let’s start with the basics. you can ﬁnd the latest version of the documentation in LTEX format in the Snort CVS repository at /doc/snort_manual. sniffer mode).org>.1 Getting Started
Snort really isn’t very hard to use./snort -v This command will run Snort and just show the IP and TCP/UDP/ICMP headers.
1. try the following: .org> and now is maintained by the Snort Team.Chapter 1
Snort Overview
This manual is based on Writing Snort Rules by Martin Roesch and further work from Chris Green <cmg@snort. If you have a better way to say something or ﬁnd that something in the documentation is outdated.e. This ﬁle aims to make using Snort easier for new users. If you just want to print out the TCP/IP packet headers to the screen (i. showing the data link layer headers. Before we proceed. drop us a line and we will update A it. which logs the packets to disk. but there are a lot of command line options to play with. • Network Intrusion Detection System (NIDS) mode. Snort can be conﬁgured to run in three modes: • Sniffer mode. the most complex and conﬁgurable conﬁguration. do this: 9
. If you want to see the application data in transit. try this: .tex. there are a few basic concepts you should understand about Snort. and it’s not always obvious which ones go together well. It was then maintained by Brian Caswell <bmc@snort. Small documentation updates are the easiest way to help out the Snort Project. If you want an even more descriptive display. which allows Snort to analyze network trafﬁc for matches against a user-deﬁned rule set and performs several actions based upon what it sees. If you would like to submit patches for this document. nothing else.2 Sniffer Mode
First. which simply reads the packets off of the network and displays them for you in a continuous stream on the console (screen). • Packet Logger mode.
1./snort -vd This instructs Snort to display the packet data as well as the headers.

3 Packet Logger Mode
OK.0 class C network. these switches may be divided up or smashed together in any combination./log. Once the packets have been logged to the binary ﬁle. For example./log Of course.1.0/24 This rule tells Snort that you want to print out the data link and TCP/IP headers as well as application data into the directory .1) host./snort -l . and you want to log the packets relative to the 192.168. We don’t need to specify a home network any longer because binary mode logs everything into a single ﬁle./snort -dev -l ./snort -d -v -e and it would do the same thing. the source address. you can read the packets back out of the ﬁle with any sniffer that supports the tcpdump binary format (such as tcpdump or Ethereal). this assumes you have a directory named log in the current directory. If you don’t. which eliminates the need to tell it how to format the output directory structure. you may notice that Snort sometimes uses the address of the remote computer as the directory in which it places packets and sometimes it uses the local host address. you don’t need to run in verbose mode or specify the -d or -e switches because in binary mode the entire packet is logged. but if you want to record the packets to the disk. not just sections of it. If you’re on a high speed network or you want to log the packets into a more compact form for later analysis. If you just specify a plain -l switch./log -h 192. In order to log relative to the home network. in the case of a tie. Snort can also read the packets back by using the -r switch.168./log -b Note the command line changes here.
! △NOTE both the source and destination hosts are on the home network. with the directory names being based on the address of the remote (non-192. you can try something like this: 10
. Snort will exit with an error message./snort -vde (As an aside. Additionally.168. Binary mode logs the packets in tcpdump format to a single binary ﬁle in the logging directory: . if you wanted to run a binary log ﬁle through Snort in sniffer mode to dump the packets to the screen.)
1. it collects every packet it sees and places it in a directory hierarchy based upon the IP address of one of the hosts in the datagram. all of these commands are pretty cool. you need to tell Snort which network is the home network: . All incoming packets will be recorded into subdirectories of the log directory. The last command could also be typed out as: . you should consider logging in binary mode. All you really need to do to place Snort into logger mode is to specify a logging directory at the command line using the -l switch—the -b binary logging switch merely provides a modiﬁer that tells Snort to log the packets in something other than the default output format of plain ASCII text. When Snort runs in this mode. you need to specify a logging directory and Snort will automatically know to go into packet logger mode: .1./snort -dev -l .. they are logged to a directory Note that if
with a name based on the higher of the two port numbers or. Packets from any tcpdump formatted ﬁle can be processed through Snort in any of its run modes. which puts it into playback mode.

0/24 -l ./log -h 192.conf This will conﬁgure Snort to run in its most basic NIDS form.. Alert modes are somewhat more complex. It’s also not necessary to record the data link headers for most applications./snort -dv -r packet. If you don’t specify an output directory for the program. try this: .log You can manipulate the data in the ﬁle in a number of ways through Snort’s packet logging and intrusion detection modes.conf where snort. The default logging and alerting mechanisms are to log in decoded ASCII format and use full alerts.1 NIDS Mode Output Options
There are a number of ways to conﬁgure the output of Snort in NIDS mode. fast. These options are: Option -A fast -A full -A -A -A -A unsock none console cmg Description Fast alert mode. socket. cmg. Generates “cmg style” alerts.0/24 -c snort. One thing to note about the last command line is that if Snort is going to be used in a long term way as an IDS.1. source and destination IPs/ports. The screen is a slow place to write data to.4. it will default to /var/log/snort. Writes the alert in a simple format with a timestamp. logging packets that trigger rules speciﬁed in the snort.
1. Sends alerts to a UNIX socket that another program can listen on. This will apply the rules conﬁgured in the snort. console. Six of these modes are accessed with the -A command line switch./log -c snort.conf is the name of your snort conﬁguration ﬁle. as well as with the BPF interface that’s available from the command line.conf ﬁle to each packet to decide if an action based upon the rule type in the ﬁle should be taken. too.conf in plain ASCII to disk using a hierarchical directory structure (just like packet logger mode). For example. Full alert mode.168. syslog. the -v switch should be left off the command line for the sake of speed. as well as two logging facilities. if you only wanted to see the ICMP packets from the log ﬁle. so you can usually omit the -e switch. Sends “fast-style” alerts to the console (screen)./snort -dev -l .1. This is the default alert mode and will be used automatically if you do not specify a mode. and none.4 Network Intrusion Detection System Mode
To enable Network Intrusion Detection System (NIDS) mode so that you don’t record every single packet sent down the wire. simply specify a BPF ﬁlter at the command line and Snort will only see the ICMP packets in the ﬁle: . There are seven alert modes available at the command line: full./snort -dvr packet./snort -d -h 192. There are several other alert output modes available at the command line. The full alert mechanism prints out the alert message in addition to the full packet headers.168. read the Snort and tcpdump man pages. .
11
. alert message.log icmp For more info on how to use the BPF interface.
1. Turns off alerting. and packets can be dropped while writing to the display.

1. please see etc/gen-msg.168. use the -s switch.0/24
1. For example. Rule-based SIDs are written directly into the rules with the sid option. 56 represents a T/TCP event. In this case./log -h 192. This will log packets in tcpdump format and produce minimal alerts. The default facilities for the syslog alerting mechanism are LOG AUTHPRIV and LOG ALERT. but still somewhat fast. In this case. If you want to conﬁgure other facilities for syslog output. try using binary logging with the “fast” output mechanism. The third number is the revision ID. See Section 2.2 Understanding Standard Alert Output
When Snort generates an alert message. it will usually look like the following: [**] [116:56:1] (snort_decoder): T/TCP Detected [**] The ﬁrst number is the Generator ID. This number is primarily used when writing signatures.3 High Performance Conﬁguration
If you want Snort to go fast (like keep up with a 1000 Mbps connection). see Section 2./snort -c snort.168./snort -c snort. please read etc/generators in the Snort source.conf
12
.
To send alerts to syslog. we know that this event came from the “decode” (116) component of Snort.1 for more details on conﬁguring syslog output.map. use the following command line to log to the default facility in /var/log/snort and send alerts to a fast alert ﬁle: . use the output plugin directives in snort.conf -A fast -h 192.6. The second number is the Snort ID (sometimes referred to as Signature ID). For a list of GIDs.conf.0/24 -s As another example. For a list of preprocessor SIDs. This allows Snort to log alerts in a binary form as fast as possible while another program performs the slow actions. use the -N command line switch.1.conf -l .1.6. For output modes available through the conﬁguration ﬁle. this tells the user what component of Snort generated this alert.Packets can be logged to their default decoded ASCII format or to a binary log ﬁle via the -b command line switch. For example: .
! △NOTE
Command line logging options override any output options speciﬁed in the conﬁguration ﬁle. If you want a text ﬁle that’s easily parsed. as each rendition of the rule should increment this number with the rev option. such as writing to a database. This allows debugging of conﬁguration issues quickly via the command line.4. you need to use uniﬁed logging and a uniﬁed log reader such as barnyard. To disable packet logging altogether.4. use the following command line to log to default (decoded ASCII) facility and send alerts to syslog: ./snort -b -A fast -c snort.

regardless of the use of --process-all-events. please refer to the --alert-before-pass option. • --process-all-events option causes Snort to process every event associated with a packet.
! △NOTE
Pass rules are special cases here. in that the event processing is terminated when a pass rule is encountered.1. • --treat-drop-as-alert causes drop and reject rules and any associated alerts to be logged as alerts. while taking the actions based on the rules ordering.4.4 Changing Alert Order
The default way in which Snort applies its rules to packets may not be appropriate for all installations. The sdrop rules are not loaded.
! △NOTE
Sometimes an errant pass rule could cause alerts to not show up. only the events for the ﬁrst action based on rules ordering are processed. then the Alert rules and ﬁnally. Without this option (default case).5. Log rules are applied. rather then the normal action. or Data Acquisition library. for packet I/O.
1. The DAQ replaces direct calls to PCAP functions with an abstraction layer that facilitates operation on a variety of hardware and software interfaces without requiring changes to Snort. you can run Snort just as you always did for ﬁle readback or snifﬁng an interface. • --alert-before-pass option forces alert rules to take affect in favor of a pass rule. This allows use of an inline policy with passive/IDS mode. For more information.5 Packet Acquisition
Snort 2. The Pass rules are applied ﬁrst. then the Drop rules.
1. you can select and conﬁgure the DAQ when Snort is invoked as follows: ./snort \ [--daq <type>] \ [--daq-mode <mode>] \ [--daq-dir <dir>] \ [--daq-var <var>] config config config config daq: <type> daq_dir: <dir> daq_var: <var> daq_mode: <mode>
<type> ::= pcap | afpacket | dump | nfq | ipq | ipfw
13
.9 introduces the DAQ.
Several command line options are available to change the order in which rule actions are taken. etc.1 Conﬁguration
Assuming that you did not disable static modules or change the default DAQ type. However. in which case you can change the default ordering to allow Alert rules to be applied before Pass rules. It is possible to select the DAQ type and mode when invoking Snort to perform PCAP readback or inline operation.

lanl. the command line overrides the conf./snort -r <file> . and directory may be speciﬁed either via the command line or in the conf ﬁle. by using a shared memory ring buffer. variable.2 PCAP
pcap is the default DAQ. On Ethernet. version. and if that hasn’t been set. This applies to static and dynamic versions of the same library. -r will force it to read-ﬁle./snort --daq pcap --daq-var buffer_size=<#bytes> Note that the pcap DAQ does not count ﬁltered packets./snort --daq pcap --daq-mode read-file -r <file> You can specify the buffer size pcap uses with: . the maximum size is 32768.<mode> ::= read-file | passive | inline <var> ::= arbitrary <name>=<value> passed to DAQ <dir> ::= path where to look for DAQ module so’s The DAQ type. DAQ type may be speciﬁed at most once in the conf and once on the command line. Also. mode. libpcap is able to queue packets into a shared buffer that Snort is able to read directly. MMAPed pcap On Linux.
14
./snort --daq pcap --daq-mode passive -i <device> . This feature is not available in the conf. since there is no conﬂict. The shared memory ring buffer libpcap can be downloaded from his website at http://public. By using PCAP FRAMES=max. -Q and –daq-mode inline are allowed. . These are equivalent: ./snort -i <device> . According to Phil. -Q will force it to inline. Once Snort linked against the shared memory libpcap.gov/cpw/. libpcap will automatically use the most frames possible. a modiﬁed version of libpcap is available that implements a shared memory ring buffer. PCAP FRAMES is the size of the ring buffer. but -Q and any other DAQ mode will cause a fatal error at start-up. this ends up being 1530 bytes per frame. Note that if Snort ﬁnds multiple versions of a given library. the mode defaults to passive.5. if snort is run w/o any DAQ arguments. Phil Woods (cpw@lanl. for a total of around 52 Mbytes of memory for the ring buffer alone. as this appears to be the maximum number of iovecs the kernel can handle. the most recent version is selected.gov) is the current maintainer of the libpcap implementation of the shared memory ring buffer. and if that hasn’t been set. You may include as many variables and directories as needed by repeating the arg / conﬁg./snort [--daq-list <dir>] The above command searches the speciﬁed directory for DAQ modules and prints type. it will operate as it always did using this module. enabling the ring buffer is done via setting the environment variable PCAP FRAMES. and attributes of each. If the mode is not set explicitly. Instead of the normal mechanism of copying the packets from kernel memory into userland memory. if conﬁgured in both places. This change speeds up Snort by limiting the number of times the packet is copied before Snort gets to perform its detection upon it.
1.

Assuming the default packet memory with a snaplen of 1518. default is ip4 <qid> ::= 0. here’s why. 2. The smallest block size that can ﬁt at least one frame is 4 KB = 4096 bytes @ 2 frames per block. 3./snort --daq nfq \ [--daq-var device=<dev>] \ [--daq-var proto=<proto>] \ [--daq-var queue=<qid>] \ [--daq-var queue_len=<qlen>] <dev> ::= ip | eth0. default is 0 Notes on iptables are given below..5 MB. You can change this with: --daq-var buffer_size_mb=<#MB> Note that the total allocated is actually higher.4 NFQ
NFQ is the new and improved way to process iptables packets: .65535./snort --daq afpacket -i <device> [--daq-var buffer_size_mb=<#MB>] [--daq-var debug] If you want to run afpacket in inline mode. Actual memory allocated is 42366 * 4 KB = 165. As a result. 4. where each member of a pair is separated by a single colon and each pair is separated by a double colon like this: eth0:eth1 or this: eth0:eth1::eth2:eth3 By default. default is 0 <qlen> ::= 0.5. The frame size is 1518 (snaplen) + the size of the AFPacket header (66 bytes) = 1584 bytes. the afpacket DAQ allocates 128MB for packet memory. you must set device to one or more interface pairs. The number of frames is 128 MB / 1518 = 84733.
1.1. 5. we need 84733 / 2 = 42366 blocks.65535.3 AFPACKET
afpacket functions similar to the memory mapped pcap DAQ but no external library is required: . default is IP injection <proto> ::= ip4 | ip6 | ip*. the numbers break down like this: 1.
15
.. etc.5.

5. default is 8000 * IPFW only supports ip4 trafﬁc.
1. It therefore does not count ﬁltered packets. etc. It replaces the inline version available in pre-2.5.9 Snort like injection and normalization. default is IP injection <proto> ::= ip4 | ip6.5 IPQ
IPQ is the old way to process iptables packets. You can optionally specify a different name./configure --enable-inline / -DGIDS Start the IPQ DAQ as follows: . start Snort like this: ./snort -i <device> -Q --daq dump --daq-var load-mode=passive 16
.pcap will be created containing all packets that passed through or were generated by snort./snort -i <device> --daq dump .65535./configure --enable-ipfw / -DGIDS -DIPFW This command line argument is no longer supported: .5..9 versions built with this: .6 IPFW
IPFW is available for BSD systems.7 Dump
The dump DAQ allows you to test the various inline mode features available in 2. It replaces the inline version available in pre-2. Furthermore. default is ip4 Notes on iptables are given below./snort --daq dump --daq-var file=<name> dump uses the pcap daq for packet acquisition./snort -r <pcap> --daq dump By default a ﬁle named inline-out. ./snort --daq ipq \ [--daq-var device=<dev>] \ [--daq-var proto=<proto>] \ <dev> ::= ip | eth0.1./snort -r <pcap> -Q --daq dump --daq-var load-mode=read-file . Note that the dump DAQ inline mode is not an actual inline mode.
1. you will probably want to have the pcap DAQ acquire in another mode like this: .9 versions built with this: ./snort -J <port#> Instead. ./snort --daq ipfw [--daq-var port=<port>] <port> ::= 1.

Use --pcap-no-filter to delete ﬁlter for following --pcap-file or --pcap-dir arguments or specify --pcap-filter again to forget previous ﬁlter and to apply to following --pcap-file or --pcap-dir arguments. This can be useful for testing and debugging Snort.pcap $ snort --pcap-single=foo.1 Command line arguments
Any of the below can be speciﬁed multiple times on the command line (-r included) and in addition to other Snort command line options. • Block packets Snort did not forward.8 Statistics Changes
The Packet Wire Totals and Action Stats sections of Snort’s output include additional ﬁelds: • Filtered count of packets ﬁltered out and not handed to Snort for analysis. that specifying --pcap-reset and --pcap-show multiple times has the same effect as specifying them once. Snort will read and analyze the packets as if they came off the wire. without this option.5.2 Examples
Read a single pcap $ snort -r foo. Can specify path to pcap or directory to recurse to get pcaps. A space separated list of pcaps to read. Reset to use no ﬁlter when getting pcaps from ﬁle or directory. A directory to recurse to look for pcaps.6.
--pcap-no-filter --pcap-reset --pcap-show
1.
1. Shell style ﬁlter to apply when getting pcaps from ﬁle or directory. Note. Added for completeness. however. The default.pcap 17
. • Replace packets Snort modiﬁed. Sorted in ASCII order. Option -r <file> --pcap-single=<file> --pcap-file=<file> --pcap-list="<list>" --pcap-dir=<dir> --pcap-filter=<filter> Description Read a single pcap. • Blacklist packets that caused Snort to block a ﬂow from passing. eg due to a block rule. The action stats show ”blocked” packets instead of ”dropped” packets to avoid confusion between dropped packets (those Snort didn’t actually see) and blocked packets (those Snort did not allow to pass).1. If reading multiple pcaps. is not to reset state. you can give it a packet capture to read.6 Reading Pcaps
Instead of having Snort listen on an interface. • Allow packets Snort analyzed and did not take action on. File that contains a list of pcaps to read.e. This ﬁlter will apply to any --pcap-file or --pcap-dir arguments following.6. • Ignore packets that caused Snort to allow a ﬂow to pass w/o inspection by this instance of Snort. i. Same as -r. • Injected packets Snort generated and sent.
1. • Whitelist packets that caused Snort to allow a ﬂow to pass w/o inspection by any analysis program. eg TCP resets. Print a line saying what pcap is currently being read. reset snort to post-conﬁguration state before reading next pcap.

cap” will be applied to ﬁles found under /home/foo/pcaps2.txt. 18
.pcap.txt This will read foo1. $ snort --pcap-filter="*.pcap foo3.pcap" --pcap-file=foo. the ﬁrst ﬁlter will be applied to foo. so all ﬁles found under /home/foo/pcaps will be included.cap” will cause the ﬁrst ﬁlter to be forgotten and then applied to the directory /home/foo/pcaps.pcap”. the ﬁrst ﬁlter will be applied to foo.pcap foo2. foo2.pcap.txt \ > --pcap-no-filter --pcap-dir=/home/foo/pcaps In this example.pcap foo2.pcap and all ﬁles under /home/foo/pcaps.txt \ > --pcap-no-filter --pcap-dir=/home/foo/pcaps \ > --pcap-filter="*. the ﬁrst ﬁlter ”*. then no ﬁlter will be applied to the ﬁles found under /home/foo/pcaps.pcap --pcap-file=foo. Read pcaps under a directory $ snort --pcap-dir="/home/foo/pcaps" This will include all of the ﬁles under /home/foo/pcaps.txt $ snort --pcap-filter="*.txt” (and any directories that are recursed in that ﬁle). Read pcaps from a command line list $ snort --pcap-list="foo1.pcap --pcap-file=foo.pcap" This will read foo1.pcap and foo3. Using ﬁlters $ cat foo. then no ﬁlter will be applied to the ﬁles found under /home/foo/pcaps. so only ﬁles ending in ”. then the ﬁlter ”*. any ﬁle ending in ”.pcap foo2. $ snort --pcap-filter="*. in other words.txt foo1.pcap" --pcap-dir=/home/foo/pcaps The above will only include ﬁles that match the shell pattern ”*.txt foo1.txt \ > --pcap-filter="*. The addition of the second ﬁlter ”*.cap" --pcap-dir=/home/foo/pcaps2 In this example.txt.pcap /home/foo/pcaps $ snort --pcap-filter="*.cap" --pcap-dir=/home/foo/pcaps In the above. foo2.pcap --pcap-file=foo.pcap” will only be applied to the pcaps in the ﬁle ”foo.pcap /home/foo/pcaps $ snort --pcap-file=foo.cap” will be included from that directory. Note that Snort will not try to determine whether the ﬁles under that directory are really pcap ﬁles or not.pcap”.Read pcaps from a ﬁle $ cat foo. so all ﬁles found under /home/foo/pcaps will be included.pcap. $ snort --pcap-filter="*.

1. The way this is counted varies per DAQ so the DAQ documentation should be consulted for more info. statistics reset. This does not include all possible output data. If you are reading pcaps. etc. minutes. The others are summarized below. • Injected packets are the result of active response which can be conﬁgured for inline or passive modes. Example: =============================================================================== Run time for packet processing was 175. The rates are based on whole seconds. meaning all buffers will be ﬂushed.Resetting state $ snort --pcap-dir=/home/foo/pcaps --pcap-reset The above example will read all of the ﬁles under /home/foo/pcaps. • Filtered packets are not shown for pcap DAQs. It includes total seconds and packets as well as packet processing rates. just the basics. Snort will be reset to a post-conﬁguration state.000%) 19
.7. Snort ran for 0 days 0 hours 2 minutes 55 seconds Pkts/min: 1858011 Pkts/sec: 21234 ===============================================================================
1.1 Timing Statistics
This section provides basic timing statistics.856509 seconds Snort processed 3716022 packets.7 Basic Output
Snort does a lot of work and outputs some useful statistics when it is done. Many of these are self-explanatory. but after each pcap is read. the totals are for all pcaps combined. unless you use –pcap-reset.7. etc. • Outstanding indicates how many packets are buffered awaiting processing.2 Packet I/O Totals
This section shows basic packet acquisition and injection peg counts obtained from the DAQ.
1. Printing the pcap $ snort --pcap-dir=/home/foo/pcaps --pcap-show The above example will read all of the ﬁles under /home/foo/pcaps and will print a line indicating which pcap is currently being read. in which case it is shown per pcap. and only shown when non-zero. it will be like Snort is seeing trafﬁc for the ﬁrst time. Example: =============================================================================== Packet I/O Totals: Received: 3716022 Analyzed: 3716022 (100. For each pcap.

1.7.4 Actions, Limits, and Verdicts
Action and verdict counts show what Snort did with the packets it analyzed. This information is only output in IDS mode (when snort is run with the -c <conf> option). • Alerts is the number of activate, alert, and block actions processed as determined by the rule actions. Here block includes block, drop, and reject actions. Limits arise due to real world constraints on processing time and available memory. These indicate potential actions that did not happen: • Match Limit counts rule matches were not processed due to the config detection: setting. The default is 5. max queue events max queue

• Queue Limit counts events couldn’t be stored in the event queue due to the config event queue: setting. The default is 8. • Log Limit counts events were not alerted due to the config event queue: • Event Limit counts events not alerted due to event filter limits. Verdicts are rendered by Snort on each packet: • Allow = packets Snort analyzed and did not take action on.

log setting. The default is 3.

• Block = packets Snort did not forward, eg due to a block rule. ”Block” is used instead of ”Drop” to avoid confusion between dropped packets (those Snort didn’t actually see) and blocked packets (those Snort did not allow to pass). • Replace = packets Snort modiﬁed, for example, due to normalization or replace rules. This can only happen in inline mode with a compatible DAQ. • Whitelist = packets that caused Snort to allow a ﬂow to pass w/o inspection by any analysis program. Like blacklist, this is done by the DAQ or by Snort on subsequent packets. • Blacklist = packets that caused Snort to block a ﬂow from passing. This is the case when a block TCP rule ﬁres. If the DAQ supports this in hardware, no further packets will be seen by Snort for that session. If not, snort will block each packet and this count will be higher. • Ignore = packets that caused Snort to allow a ﬂow to pass w/o inspection by this instance of Snort. Like blacklist, this is done by the DAQ or by Snort on subsequent packets. 21

1.8 Tunneling Protocol Support
Snort supports decoding of GRE, IP in IP and PPTP. To enable support, an extra conﬁguration option is necessary: $ ./configure --enable-gre To enable IPv6 support, one still needs to use the conﬁguration option: $ ./configure --enable-ipv6

1.8.1 Multiple Encapsulations
Snort will not decode more than one encapsulation. Scenarios such as Eth IPv4 GRE IPv4 GRE IPv4 TCP Payload or Eth IPv4 IPv6 IPv4 TCP Payload will not be handled and will generate a decoder alert.

Decoding of PPTP, which utilizes GRE and PPP, is not currently supported on architectures that require word alignment such as SPARC.

1.9 Miscellaneous
1.9.1 Running Snort as a Daemon
If you want to run Snort as a daemon, you can the add -D switch to any combination described in the previous sections. Please notice that if you want to be able to restart Snort by sending a SIGHUP signal to the daemon, you must specify the full path to the Snort binary when you start it, for example: /usr/local/bin/snort -d -h 192.168.1.0/24 \ -l /var/log/snortlogs -c /usr/local/etc/snort.conf -s -D Relative paths are not supported due to security concerns. Snort PID File When Snort is run as a daemon , the daemon creates a PID ﬁle in the log directory. In Snort 2.6, the --pid-path command line switch causes Snort to write the PID ﬁle in the directory speciﬁed. Additionally, the --create-pidfile switch can be used to force creation of a PID ﬁle even when not running in daemon mode. The PID ﬁle will be locked so that other snort processes cannot start. Use the --nolock-pidfile switch to not lock the PID ﬁle.

1.9.2 Running in Rule Stub Creation Mode
If you need to dump the shared object rules stub to a directory, you might need to use the –dump-dynamic-rules option. These rule stub ﬁles are used in conjunction with the shared object rules. The path can be relative or absolute. /usr/local/bin/snort -c /usr/local/etc/snort.conf \ --dump-dynamic-rules=/tmp This path can also be conﬁgured in the snort.conf using the conﬁg option dump-dynamic-rules-path as follows: config dump-dynamic-rules-path: /tmp/sorules The path conﬁgured by command line has precedence over the one conﬁgured using dump-dynamic-rules-path.

23

You can also combine the -O switch with the -h switch to only obfuscate the IP addresses of hosts on the home network. Users can specify either a decimal value (-G 1) or hex value preceded by 0x (-G 0x11). allowing evaluation of inline behavior without affecting trafﬁc.0/24 class C network: . Snort can be conﬁgured to run in inline-test mode using the command line option (–enable-inline-test) or using the snort conﬁg option policy mode as follows:
24
. or on the same CPU but a different interface. inline.9.4. This option can be used when running multiple instances of snort. Explanation of Modes • Inline When Snort is in Inline mode. Each Snort instance will use the value speciﬁed to generate unique event IDs. obfuscating only the addresses from the 192.9. you might want to use the -O switch.conf \ --dump-dynamic-rules snort. Snort policies can be conﬁgured in these three modes too.168.1. This is also supported via a long option --logid. the -G command line option was added that speciﬁes an instance identiﬁer for the event logs. Snort can be conﬁgured to passive mode using the snort conﬁg option policy mode as follows: config policy_mode:tap • Inline-Test Inline-Test mode simulates the inline mode of snort. For example. This switch obfuscates your IP addresses in packet printouts.
1.1.
1.4 Specifying Multiple-Instance Identiﬁers
In Snort v2. This is handy if you don’t want people on the mailing list to know the IP addresses involved. either on different CPUs. This is useful if you don’t care who sees the address of the attacking host. Drop rules are not loaded (without –treat-drop-as-alert).168.0/24
1.5 Snort Modes
Snort can operate in three different modes namely tap (passive). you could use the following command to read the packets from a log ﬁle and dump them to the screen.9.conf: config dump-dynamic-rules-path: /tmp/sorules In the above mentioned scenario the dump path is set to /tmp/sorules. it acts as a IDS. it acts as an IPS allowing drop rules to trigger.log -O -h 192. and inline-test./snort -d -v -r snort. Snort can be conﬁgured to run in inline mode using the command line argument -Q and snort conﬁg option policy mode as follows: snort -Q config policy_mode:inline • Passive When Snort is in Passive mode. The drop rules will be loaded and will be triggered as a Wdrop (Would Drop) alert./usr/local/bin/snort -c /usr/local/etc/snort.3 Obfuscating IP Address Printouts
If you need to post packet logs to public mailing lists.

10.
2.1 Includes
The include keyword allows other snort conﬁg ﬁles to be included within the snort. See Section 2.conf indicated on the Snort command line.1.1.1024:1050] ipvar MY_NET [192. use a regular ’var’.1. reading the contents of the named ﬁle and adding the contents in the place where the include statement appears in the ﬁle. It works much like an #include from the C programming language. msg:"SYN packet".0/24] alert tcp any any -> $MY_NET $MY_PORTS (flags:S.rule 26
.168.80.) include $RULE_PATH/example. or portvar keywords as follows: var RULES_PATH rules/ portvar MY_PORTS [22.1 Format
include <include file path/name>
! △NOTE is no semicolon at the end of this line.2 Variables
Three types of variables may be deﬁned in Snort: • var • portvar • ipvar
! △NOTE are only enabled with IPv6 support.1.
2.2 for more information on deﬁning and using variables in Snort conﬁg ﬁles. ipvar. Note: ’ipvar’s
These are simple substitution variables set with the var. Note that there
Included ﬁles will substitute any predeﬁned variable values into their own variable references.Chapter 2
Conﬁguring Snort
2.0/24.1. Without IPv6 support.1.

Use of !any: ipvar EXAMPLE any alert tcp !$EXAMPLE any -> any any (msg:"Example".1. or any combination of the three. Negation is handled differently compared with Snort versions 2.![2.0/24. ’any’ will specify any ports.1.2. Valid port ranges are from 0 to 65535.0/16] Port Variables and Port Lists Portlists supports the declaration and lookup of ports and the representation of lists and ranges of ports.2.2.2 and 2.0/24.2. IP lists.0/8. If IPv6 support is enabled.1.2.1. sid:1. but it will be deprecated in a future release.sid:3.2. negated IP ranges that are more general than non-negated IP ranges are not allowed.sid:2.) Different use of !any: ipvar EXAMPLE !any alert tcp $EXAMPLE any -> any any (msg:"Example".2.1. each element in a list was logically OR’ed together.255.1.2.1.3]] The order of the elements in the list does not matter.2.IP Variables and IP Lists IPs may be speciﬁed individually. Variables. IP variables should be speciﬁed using ’ipvar’ instead of ’var’.) The following examples demonstrate some invalid uses of IP variables and IP lists.!1. IPs.1. or lists may all be negated with ’!’.2.0.x and earlier. Previously. such as in: [10:50.7.2. Lists of ports must be enclosed in brackets and port ranges may be speciﬁed with a ’:’.![2.0/24.2.1.2.2. IP lists now OR non-negated elements and AND the result with the OR’ed negated elements. with the exception of IPs 2.2. ranges.1.2.1] Nonsensical negations: ipvar EXAMPLE [1.!1. The element ’any’ can be used to match all IPs.0. although ’!any’ is not allowed.2.2.1. Also.888:900]
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. ipvar EXAMPLE [1. in a list. The following example list will match the IP 1.1.3.3]] alert tcp $EXAMPLE any -> any any (msg:"Example". and CIDR blocks may be negated with ’!’.1.1.1.2.2.2.2.2.1.2.) alert tcp [1.1 and IP from 2.2.0 to 2. as a CIDR block.1.) Logical contradictions: ipvar EXAMPLE [1. [1. Using ’var’ for an IP variable is still allowed for backward compatibility. but ’!any’ is not allowed.0/24] any -> any any (msg:"Example".2.1. Also.0. See below for some valid examples if IP variables and IP lists.!1.sid:3.

) Port variable used as an IP: alert tcp $EXAMPLE1 any -> any any (msg:"Example". For backwards compatibility. You can deﬁne meta-variables using the $ operator. a ’var’ can still be used to declare a port variable. provided the variable name either ends with ’ PORT’ or begins with ’PORT ’.) alert tcp any $PORT_EXAMPLE2 -> any any (msg:"Example".9999:20000] (msg:"Example". These can be used with the variable modiﬁer operators ? and -. sid:2. portvar EXAMPLE1 80 var EXAMPLE2_PORT [80:90] var PORT_EXAMPLE2 [1] portvar EXAMPLE3 any portvar EXAMPLE4 [!70:90] portvar EXAMPLE5 [80. The use of ’var’ to declare a port variable will be deprecated in a future release. sid:5.!80] Ports out of range: portvar EXAMPLE7 [65536] Incorrect declaration and use of a port variable: var EXAMPLE8 80 alert tcp any $EXAMPLE8 -> any any (msg:"Example". sid:4. sid:1.100:200] alert tcp any $EXAMPLE1 -> any $EXAMPLE2_PORT (msg:"Example". sid:3.Port variables should be speciﬁed using ’portvar’.) Several invalid examples of port variables and port lists are demonstrated below: Use of !any: portvar EXAMPLE5 !any var EXAMPLE5 !any Logical contradictions: portvar EXAMPLE6 [80. as described in the following table:
28
.) alert tcp any 90 -> any [100:1000.91:95. The following examples demonstrate several valid usages of both port variables and port lists.) Variable Modiﬁers Rule variable names can be modiﬁed in several ways.

168. Valid embedded variable: portvar pvar1 80 portvar pvar2 [$pvar1.90] Likewise.3 Conﬁg
Many conﬁguration and command line options of Snort can be speciﬁed in the conﬁguration ﬁle. Replaces with the contents of variable var.Variable Syntax var $(var) or $var $(var:-default) $(var:?message)
Description Deﬁnes a meta-variable.0/24 log tcp any any -> $(MY_NET:?MY_NET is undefined!) 23 Limitations When embedding variables. They should be renamed instead: Invalid redeﬁnition: var pvar 80 portvar pvar 90
2.
Here is an example of advanced variable usage in action: ipvar MY_NET 192. types can not be mixed. but old-style variables (with the ’var’ keyword) can not be embedded inside a ’portvar’. Replaces with the contents of variable var or prints out the error message and exits.1.1. variables can not be redeﬁned if they were previously deﬁned as a different type.90] Invalid embedded variable: var pvar1 80 portvar pvar2 [$pvar1. port variables can be deﬁned in terms of other port variables. Replaces the contents of the variable var with “default” if var is undeﬁned. Format config <directive> [: <value>]
29
. For instance.

Description Appends interface name to alert (snort -I). Sets the alerts output ﬁle. Speciﬁes the maximum number of nodes to track when doing ASN1 decoding. See Section 3.5.31 for more information and examples. If Snort was conﬁgured to enable decoder and preprocessor rules, this option will cause Snort to revert back to it’s original behavior of alerting if the decoder or preprocessor generates an event. Speciﬁes BPF ﬁlters (snort -F). Types of packets to drop if invalid checksums. Values: none, noip, notcp, noicmp, noudp, ip, tcp, udp, icmp or all (only applicable in inline mode and for packets checked per checksum mode conﬁg option). Types of packets to calculate checksums. Values: none, noip, notcp, noicmp, noudp, ip, tcp, udp, icmp or all. Chroots to speciﬁed dir (snort -t). See Table 3.2 for a list of classiﬁcations. Forks as a daemon (snort -D). Decodes Layer2 headers (snort -e). Global conﬁguration directive to enable or disable the loading of rules into the detection engine. Default (with or without directive) is enabled. Specify disabled to disable loading rules. Selects the type of DAQ to instantiate. The DAQ with the highest version of the given type is selected if there are multiple of the same type (this includes any built-in DAQs). Select the DAQ mode: passive, inline, or read-ﬁle. Not all DAQs support modes. See the DAQ distro README for possible DAQ modes or list DAQ capabilities for a brief summary. Set a DAQ speciﬁc variable. Snort just passes this information down to the DAQ. See the DAQ distro README for possible DAQ variables. Tell Snort where to look for available dynamic DAQ modules. This can be repeated. The selected DAQ will be the one with the latest version. Tell Snort to dump basic DAQ capabilities and exit. You can optionally specify a directory to include any dynamic DAQs from that directory. You can also preceed this option with extra DAQ directory options to look in multiple directories.

Other options that affect fast pattern matching. • split-any-any – A memory/performance tradeoff. By default, ANYANY port rules are added to every non ANY-ANY port group so that only one port group rule evaluation needs to be done per packet. Not putting the ANY-ANY port rule group into every other port group can signiﬁcantly reduce the memory footprint of the fast pattern matchers if there are many ANYANY port rules. But doing so may require two port group evaluations per packet - one for the speciﬁc port group and one for the ANY-ANY port group, thus potentially reducing performance. This option is generic and can be used with any search-method but was speciﬁcally intended for use with the ac search-method where the memory footprint is signiﬁcantly reduced though overall fast pattern performance is better than ac-bnfa. Of note is that the lower memory footprint can also increase performance through fewer cache misses. Default is not to split the ANY-ANY port group. • search-optimize – Optimizes fast pattern memory when used with search-method ac or ac-split by dynamically determining the size of a state based on the total number of states. When used with ac-bnfa, some fail-state resolution will be attempted, potentially increasing performance. Default is not to optimize. • max-pattern-len <integer> – This is a memory optimization that speciﬁes the maximum length of a pattern that will be put in the fast pattern matcher. Patterns longer than this length will be truncated to this length before inserting into the pattern matcher. Useful when there are very long contents being used and truncating the pattern won’t diminish the uniqueness of the patterns. Note that this may cause more false positive rule evaluations, i.e. rules that will be evaluated because a fast pattern was matched, but eventually fail, however CPU cache can play a part in performance so a smaller memory footprint of the fast pattern matcher can potentially increase performance. Default is to not set a maximum pattern length.

Other detection engine options. • no stream inserts – Speciﬁes that stream inserted packets should not be evaluated against the detection engine. This is a potential performance improvement with the idea that the stream rebuilt packet will contain the payload in the inserted one so the stream inserted packet doesn’t need to be evaluated. Default is to inspect stream inserts. • max queue events <integer> – Speciﬁes the maximum number of events to queue per packet. Default is 5 events. • enable-single-rule-group – Put all rules into one port group. Not recommended. Default is not to do this. • bleedover-port-limit – The maximum number of source or destination ports designated in a rule before the rule is considered an ANY-ANY port group rule. Default is 1024.

Snort’s packet decoder only decodes Teredo (IPv6 over UDP over IPv4) trafﬁc on UDP port 3544. there could be situations where two private networks share the same IP space and different MPLS labels are used to differentiate trafﬁc from the two VPNs. By default. UDP.4 for more information and examples. Specify the protocol (TCP. it is off. This option makes Snort decode Teredo trafﬁc on all UDP ports. or ICMP). In such a situation. When this option is off and MPLS multicast trafﬁc is detected. The default is 1024 bits and maximum is 2096. this conﬁguration option should not be turned on. Speciﬁes ports to ignore (useful for ignoring noisy NFS trafﬁc). followed by a list of ports.config enable decode oversized drops
config enable deep teredo inspection
config enable ipopt drops config enable mpls multicast
config enable mpls overlapping ip
config enable tcpopt drops config enable tcpopt experimental drops config enable tcpopt obsolete drops config enable tcpopt ttcp drops config enable ttcp drops config event filter: memcap <bytes> config event queue: [max queue <num>] [log <num>] [order events <order>]
Enable dropping packets that have headers containing length ﬁelds for which the value is greater than the length of the packet. Enables the dropping of bad packets with T/TCP option. (only applicable in inline mode). Enables support for overlapping IP addresses in an MPLS network. Enables the dropping of bad packets with bad/truncated TCP option (only applicable in inline mode). Sets the network interface (snort -i). Set global memcap in bytes for thresholding. Port ranges are supported. However. where there are no overlapping IP addresses. it is off. In a normal situation.
config flowbits size: config ignore ports: <port-list> config interface:
<num-bits> <proto>
<iface>
35
. You can use the following options: • max queue <integer> (max events supported) • log <integer> (number of events to log) • order events [priority|content length] (how to order events within the queue) See Section 2. Enables the dropping of bad packets with experimental TCP option. IP. Speciﬁes the maximum number of ﬂowbit tags that can be used within a rule set. (only applicable in inline mode). Default is 1048576 bytes (1 megabyte). enable decode oversized alerts must also be enabled for this to be effective (only applicable in inline mode). This option is needed when the network allows MPLS multicast trafﬁc. Snort will generate an alert. Speciﬁes conditions about Snort’s event queue. (only applicable in inline mode). By default. Enables the dropping of bad packets with bad/truncated IP options (only applicable in inline mode). Enables the dropping of bad packets with T/TCP option. Enables the dropping of bad packets with obsolete TCP option. this conﬁguration option should be turned on. Enables support for MPLS multicast.4. (only applicable in inline mode).

an error is logged and the remainder of the hosts are ignored. Minimum value is 32 and the maximum is 524288 (512k). Base version should be a string in all conﬁguration ﬁles including included ones.2 for more details. Disables pcre pattern matching. Default is on) • bad ipv6 frag alert on|off (Specify whether or not to alert. This option is used to avoid race conditions when modifying and loading a conﬁguration within a short time span . Default is on) • frag timeout <integer> (Specify amount of time in seconds to timeout ﬁrst frag in hash table) • max frag sessions <integer> (Specify the number of fragments to track in the hash table)
config logdir: <dir> config max attribute hosts:
<hosts>
config max mpls labelchain len: <num-hdrs> config min ttl: <ttl> config mpls payload type: ipv4|ipv6|ethernet config config config config config no promisc nolog nopcre obfuscate order: <order>
config pcre match limit: <integer>
config pcre match limit recursion: <integer>
config pkt count: <N> config policy version: <base-version-string> [<binding-version-string>]
config profile preprocs config profile rules
Sets the logdir (snort -l). eg: pass alert log activation. This option is only useful if the value is less than the pcre match limit Exits after N packets (snort -n). For example. A value of 0 results in no PCRE evaluation.7). Sets a Snort-wide minimum ttl to ignore all trafﬁc. bad ipv6 frag alert on|off] [. The default is 10000. frag timeout <secs>] [. Restricts the amount of backtracking a given PCRE option.5. Disables logging. In addition. Its default value is -1. In addition to ipv4. Changes the order that rules are evaluated. A value of -1 allows for unlimited PCRE. Print statistics on preprocessor performance. Note: Alerts will still occur. Sets a Snort-wide MPLS payload type.config ipv6 frag: [bsd icmp frag alert on|off] [. Supply versioning information to conﬁguration ﬁles. Sets a Snort-wide limit on the number of MPLS headers a packet can have.5. which means that there is no limit on label chain length. The snort default value is 1500. Print statistics on rule performance. max frag sessions <max-track>]
The following options can be used: • bsd icmp frag alert on|off (Specify whether or not to alert. Sets a limit on the maximum number of hosts to read from the attribute table.
36
. up to the PCRE library compiled limit (around 10 million). See Section 2. If the number of hosts in the attribute table exceeds this value. The default MPLS payload type is ipv4 Disables promiscuous mode (snort -p).before Snort has had a chance to load a previous conﬁguration. Obfuscates IP Addresses (snort -O). A value of 0 results in no PCRE evaluation. This option is only supported with a Host Attribute Table (see section 2. (snort -N). ipv6 and ethernet are also valid options. A value of -1 allows for unlimited PCRE. binding version must be in any ﬁle conﬁgured with config binding. Restricts the amount of stack used by a given PCRE option. it will limit the number of nested repeats within a pattern. The snort default value is 1500.1 for more details. up to the PCRE library compiled limit (around 10 million). See Section 2.

Sets UID to <id> (snort -u). Sets umask when running (snort -m). same effect as -P <snaplen> or --snaplen <snaplen> options. Note this option is only available if Snort was built to use time stats with --enable-timestats.
37
.7. inline or inline test. Setting this option to a value of 0 will disable the packet limit.5 on using the tag option when writing rules for more details.config quiet
config read bin file: config reference:
<pcap>
<ref>
config reference net <cidr>
config response: [attempts <count>] [. A value of 0 disables the memcap. the obfuscated net will be used if the packet contains an IP address in the reference net. Uses UTC instead of local time for timestamps (snort -U). Adds a new reference system to Snort. same effect as -r <tf> option. NOTE: The command line switch -q takes effect immediately after processing the command line parameters. Maximum value is the maximum value an unsigned 32 bit integer can hold which is 4294967295 or 4GB. See Section 3. Set global memcap in bytes for so rules that dynamically allocate memory for storing session data in the stream preprocessor.conf takes effect when the conﬁguration line in snort. The default value when this option is not conﬁgured is 256 packets. Set global memcap in bytes for thresholding. whereas using config quiet in snort. and the default is off.) Set the amount of time in seconds between logging time stats. from which to send responses. Causes Snort to ignore vlan headers for the purposes of connection tracking. Sets assurance mode for stream (stream is established). this option sets the maximum number of packets to be tagged regardless of the amount deﬁned by the other metric. Speciﬁes a pcap ﬁle to use (instead of reading from network). device <dev>]
config config config config
set gid: <gid> set uid: <uid> show year snaplen: <bytes> <bytes>
config so rule memcap:
config stateful config tagged packet limit: <max-tag>
config threshold:
memcap <bytes>
config timestats interval:
<secs>
config config config config
umask: <umask> utc verbose vlan agnostic
config policy mode: tap|inline|inline test
Disables banner and status reports (snort -q). Sets the policy mode to either passive. (This is deprecated. Uses verbose logging to STDOUT (snort -v). This option is only valid in the base conﬁguration when using multiple conﬁgurations. Shows year in timestamps (snort -y). These options may appear in any order but must be comma separated.com/?id= For IP obfuscation. Default is 0. Default is 1048576 bytes (1 megabyte).conf is parsed. Use conﬁg event ﬁlter instead. Default is 3600 (1 hour). Set the snaplength of packet. eg: myref http://myurl. Also used to determine how to set up the logging directory structure for the session post detection rule option and ASCII output plugin an attempt is made to name the log directories after the IP address that is not in the reference net. such as eth0. Set the number of straﬁng attempts per injected response and/or the device. Changes GID to speciﬁed GID (snort -g). The are intended for passive mode. When a metric other than packets is used in a tag option in a rule. That may occur after other conﬁguration settings that result in output to console or syslog.

but after the packet has been decoded.snort. Frag 3 Conﬁguration Frag3 conﬁguration is somewhat more complex than frag2. There can be an arbitrary number of 38
. There are at least two preprocessor directives required to activate frag3. a global conﬁguration directive and an engine instantiation. They allow the functionality of Snort to be extended by allowing users and programmers to drop modular plugins into Snort fairly easily. The idea of a target-based system is to model the actual targets on the network instead of merely modeling the protocols and looking for attacks within them. Unfortunately. it is possible to evade the IDS. if the attacker has more information about the targets on a network than the IDS does. The basic idea behind target-based IDS is that we tell the IDS information about hosts on the network so that it can avoid Ptacek & Newsham style evasion attacks based on information about how an individual target IP stack operates. The format of the preprocessor directive in the Snort conﬁg ﬁle is: preprocessor <name>: <options>
2. Vern Paxson and Umesh Shankar did a great paper on this very topic in 2003 that detailed mapping the hosts on a network and determining how their various IP stack implementations handled the types of problems seen in IP defragmentation and TCP stream reassembly.2 Preprocessors
Preprocessors were introduced in version 1. Frag3 was implemented to showcase and prototype a target-based module within Snort to test this idea.org/docs/idspaper/. Faster execution than frag2 with less complex data management. Frag3 is intended as a replacement for the frag2 defragmentation module and was designed with the following goals: 1. heavily fragmented environments the nature of the splay trees worked against the system and actually hindered performance. The frag2 preprocessor used splay trees extensively for managing the data structures associated with defragmenting packets. Check it out at http://www. As I like to say. For an IDS this is a big problem.2. Once we have this information we can start to really change the game for these complex modeling problems. In an environment where the attacker can determine what style of IP defragmentation is being used on a particular target.org/vern/papers/activemap-oak03.2.icir. Target-based host modeling anti-evasion techniques. there are ambiguities in the way that the RFCs deﬁne some of the edge conditions that may occur and when this happens different people implement certain aspects of their IP stacks differently.1 Frag3
The frag3 preprocessor is a target-based IP defragmentation module for Snort. This is where the idea for “target-based IDS” came from. Frag3 uses the sfxhash data structure and linked lists for data handling internally which allows it to have much more predictable and deterministic performance in any environment which should aid us in managing heavily fragmented environments. 2. The packet can be modiﬁed or analyzed in an out-of-band manner using this mechanism.5 of Snort. the attacker can try to fragment packets such that the target will put them back together in a speciﬁc manner while any passive systems trying to model the host trafﬁc have to guess which way the target OS is going to handle the overlaps and retransmits. Target-based analysis is a relatively new concept in network-based intrusion detection. Preprocessor code is run before the detection engine is called. For more detail on this issue and how it affects IDS. check out the famous Ptacek & Newsham paper at http://www. Splay trees are excellent data structures to use when you have some assurance of locality of reference for the data that you are handling but in high speed. When IP stacks are written for different operating systems. they are usually implemented by people who read the RFCs and then write their interpretation of what the RFC outlines into code. We can also present the IDS with topology information to avoid TTL-based evasions and a variety of other issues. but that’s a topic for another day. Preprocessors are loaded and conﬁgured using the preprocessor keyword.pdf.

bsd. Default is 4MB. This is an optional parameter. prealloc memcap.Maximum simultaneous fragments to track. – detect anomalies .Detect fragment anomalies. Fragments in the engine for longer than this period will be automatically dropped. The accepted range for this option is 1 . Default type is bsd. Default is 1.Memory cap for self preservation.engines deﬁned at startup with their own conﬁguration. – memcap <bytes> . This engine will only run for packets with destination addresses contained within the IP List. detect anomalies option must be conﬁgured for this option to take effect. Anyone who develops more mappings and would like to add to this list please feel free to send us an email!
39
. last.Minimum acceptable TTL value for a fragment packet.Option to turn off the preprocessor.255.Limits the number of overlapping fragments per packet. if detect anomalies is also conﬁgured. Fragments smaller than or equal to this limit are considered malicious and an event is raised. The default is ”0” (unlimited).IP List to bind this engine to. – max frags <number> . By default this option is turned off. The known mappings are as follows. The default is ”0” (unlimited). Global Conﬁguration • Preprocessor name: frag3 global • Available options: NOTE: Global conﬁguration options are comma separated. the minimum is ”0”.Select a target-based defragmentation mode. but only one global conﬁguration. and prealloc frags are applied when speciﬁed with the conﬁguration. bsdright. The Paxson Active Mapping paper introduced the terminology frag3 is using to describe policy types. detect anomalies option must be conﬁgured for this option to take effect. Available types are ﬁrst.Deﬁnes smallest fragment size (payload size) that should be considered valid. – policy <type> . This conﬁg option takes values equal to or greater than zero. linux. – bind to <ip list> . – disabled . When the preprocessor is disabled only the options memcap. – overlap limit <number> . This is an optional parameter. – prealloc frags <number> . – min ttl <value> . Engine Conﬁguration • Preprocessor name: frag3 engine • Available options: NOTE: Engine conﬁguration options are space separated. Default value is all. Use preallocated fragment nodes (faster in some situations). Default is 60 seconds. – timeout <seconds> . – min fragment length <number> .Timeout for fragments. Default is 8192.Alternate memory management mode.

data received outside the TCP window. \ [flush_on_alert]. etc). Anomaly Detection TCP protocol anomalies.2. other protocol normalizers/preprocessors to dynamically conﬁgure reassembly behavior as required by the application layer protocol. while others do not. FIN and Reset sequence numbers.2 Stream5
The Stream5 preprocessor is a target-based TCP reassembly module for Snort. the rule ’ﬂow’ and ’ﬂowbits’ keywords are usable with TCP as well as UDP trafﬁc. Its event output is packet-based so it will work with all output modes of Snort. such as data on SYN packets. etc are conﬁgured via the detect anomalies option to the TCP conﬁguration. With Stream5.
2. It is capable of tracking sessions for both TCP and UDP. Data on SYN. [show_rebuilt_packets]. [disabled]
41
. direction. [max_icmp <number>]. TCP Timestamps. Read the documentation in the doc/signatures directory with ﬁlenames that begin with “123-” for information on the different event types. \ [memcap <number bytes>]. For example. identify sessions that may be ignored (large data transfers. Some of these anomalies are detected on a per-target basis. Stream API Stream5 fully supports the Stream API. Stream5 Global Conﬁguration Global settings for the Stream5 preprocessor. and the policies supported by Stream5 are the results of extensive research with many target operating systems. which effectively terminate a TCP or UDP session. [max_tcp <number>]. like Frag3. UDP sessions are established as the result of a series of UDP packets from two end points via the same set of ports. Transport Protocols TCP sessions are identiﬁed via the classic TCP ”connection”. ICMP messages are tracked for the purposes of checking for unreachable and service unavailable messages. etc) that can later be used by rules. etc. and update the identifying information about the session (application protocol. [max_udp <number>]. preprocessor stream5_global: \ [track_tcp <yes|no>]. \ [track_icmp <yes|no>]. \ [prune_log_max <bytes>]. \ [track_udp <yes|no>]. Target-Based Stream5.Frag 3 Alert Output Frag3 is capable of detecting eight different types of anomalies. The methods for handling overlapping data. introduces target-based actions for handling of overlapping data and other TCP anomalies. a few operating systems allow data in TCP SYN packets.

Backwards compatibility. Flush a TCP stream when an alert is generated on that stream. \ [ignore_any_rules]. minimum is ”32768” (32KB). maximum is ”1048576”. The default is set to off. This can have multiple occurrences. [dont_reassemble_async]. The default is set to any. The default is ”no”.
Stream5 TCP Conﬁguration Provides a means on a per IP address target to conﬁgure TCP policy. [flush_factor <number segs>] Option bind to <ip addr> timeout <num seconds>
\
Description IP address or network for this policy. Track sessions for UDP. the minimum is ”1”. and that policy is not bound to an IP address or network. The default is set to off. Print a message when a session terminates that was consuming more than the speciﬁed number of bytes. per policy that is bound to an IP address or network. \ [dont_store_large_packets]. maximum is ”1073741824” (1GB). \ [protocol <client|server|both> <all|service name [service name]*>].
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. max tcp.Option track tcp <yes|no> max tcp <num sessions> memcap <num bytes> track udp <yes|no> max udp <num sessions> track icmp <yes|no> max icmp <num sessions> disabled
flush on alert show rebuilt packets prune log max <num bytes>
Description Track sessions for TCP. The default is ”yes”. Maximum simultaneous UDP sessions tracked. The default is ”30”. max udp and max icmp are applied when speciﬁed with the conﬁguration. maximum is ”1048576”. [policy <policy_id>]. \ [check_session_hijacking]. The default is ”131072”. Track sessions for ICMP. Print/display packet after rebuilt (for debugging). and the maximum is ”86400” (approximately 1 day). minimum is ”1”. When the preprocessor is disabled only the options memcap. minimum is ”1”. The default is ”8388608” (8MB). The default is ”65536”. Option to disable the stream5 tracking. [detect_anomalies]. \ [timeout <number secs>]. minimum is ”1”. The default is ”1048576” (1MB). Maximum simultaneous TCP sessions tracked. maximum is ”1048576”. \ [small_segments <number> bytes <number> [ignore_ports number [number]*]]. \ [overlap_limit <number>]. Memcap for TCP packet storage. Maximum simultaneous ICMP sessions tracked. preprocessor stream5_tcp: \ [bind_to <ip_addr>]. [ports <client|server|both> <all|number [number]*>]. One default policy must be speciﬁed. [max_window <number>]. Session timeout. \ [max_queued_bytes <bytes>]. The default is ”yes”. minimum can be either ”0” (disabled) or if not disabled the minimum is ”1024” and maximum is ”1073741824”. The default is ”262144”. By default this option is turned off. [max_queued_segs <number segs>]. [use_static_footprint_sizes]. \ [require_3whs [<number secs>]].

3 and newer Limits the number of overlapping packets per session.2 and earlier windows Windows 2000. Alerts are generated (per ’detect anomalies’ option) for either the client or server when the MAC address for one side or the other does not match. Using this option may result in missed attacks. OpenBSD 3. Detect and alert on TCP protocol anomalies. The default is set to queue packets. A message is written to console/syslog when this limit is enforced. and the maximum is ”1073725440” (65535 left shift 14). NetBSD 2.x and newer linux Linux 2. The default is set to off. Windows XP. there are no checks performed. If an ethernet layer is not part of the protocol stack received by Snort. The default is ”0” (unlimited). Establish sessions only on completion of a SYN/SYN-ACK/ACK handshake. This allows a grace period for existing sessions to be considered established during that interval immediately after Snort is started.policy <policy id>
overlap limit <number> max window <number>
require 3whs [<number seconds>]
detect anomalies check session hijacking
use static footprint sizes
dont store large packets
dont reassemble async max queued bytes <bytes>
The Operating System policy for the target OS. the minimum is ”0”. Check for TCP session hijacking. The default is set to off. Performance improvement to not queue large packets in reassembly buffer. The default is set to off.x and newer. This check validates the hardware (MAC) address from both sides of the connect – as established on the 3-way handshake against subsequent packets received on the session. Use static values for determining when to build a reassembled packet to allow for repeatable tests. bsd FresBSD 4.
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. and the maximum is ”255”. the minimum is ”0”. Windows 95/98/ME win2003 Windows 2003 Server vista Windows Vista solaris Solaris 9.x and newer hpux HPUX 11 and newer hpux10 HPUX 10 irix IRIX 6 and newer macos MacOS 10. with a non-zero minimum of ”1024”. Maximum TCP window allowed. A value of ”0” means unlimited. The default is set to off. This option should not be used production environments. The policy id can be one of the following: Policy Name Operating Systems. The default is set to off. The optional number of seconds speciﬁes a startup timeout. Limit the number of bytes queued for reassembly on a given TCP session to bytes. Don’t queue packets for reassembly if trafﬁc has not been seen in both directions. first Favor ﬁrst overlapped segment. The default is ”0” (don’t consider existing sessions established). The default is ”0” (unlimited). This option is intended to prevent a DoS against Stream5 by an attacker using an abnormally large window. so using a value near the maximum is discouraged. the minimum is ”0”. and a maximum of ”1073741824” (1GB). That is the highest possible TCP window per RFCs. and the maximum is ”86400” (approximately 1 day).x and newer. Default is ”1048576” (1MB).4 and newer old-linux Linux 2. last Favor ﬁrst overlapped segment.

or both and list of ports in which to perform reassembly. [ignore_any_rules]
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. This can appear more than once in a given conﬁg. with a maximum of ”2048”. If only a bind to option is used with no other options that TCP policy uses all of the default values. Don’t process any -> any (ports) rules for TCP that attempt to match payload if there are no port speciﬁc rules for the src or destination port. Specify the client. This option can be used only in default policy. there should be only one occurrence of the UDP conﬁguration. Conﬁgure the maximum small segments queued. This feature requires that detect anomalies be enabled. server. This can appear more than once in a given conﬁg. derived based on an average size of 400 bytes. or byte test options. deﬁnes the list of ports in which will be ignored for this rule. A message is written to console/syslog when this limit is enforced. The default settings are ports client 21 23 25 42 53 80 110 111 135 136 137 139 143 445 513 514 1433 1521 2401 3306. The minimum port allowed is ”1” and the maximum allowed is ”65535”. ignore ports is optional. including any of the internal defaults (see 2. The second number is the minimum bytes for a segment to be considered ”small”. Using this does not affect rules that look at protocol headers. server. only those with content. Since there is no target based binding. and a maximum of ”1073741824” (1GB). A message is written to console/syslog when this limit is enforced. Rules that have ﬂow or ﬂowbits will never be ignored.max queued segs <num>
small segments <number> bytes <number> [ignore ports <number(s)> ]
ports <client|server|both> <all|number(s)>
protocol <client|server|both> <all|service name(s)>
ignore any rules
flush factor
Limit the number of segments queued for reassembly on a given TCP session.7). The default is ”2621”. The default settings are ports client ftp telnet smtp nameserver dns http pop3 sunrpc dcerpc netbios-ssn imap login shell mssql oracle cvs mysql. The default value is ”0” (disabled). The default is ”off”. with a maximum of ”2048”.
! △NOTE
If no options are speciﬁed for a given TCP policy. or both and list of services in which to perform reassembly. The number of ports can be up to ”65535”. PCRE. The ﬁrst number is the number of consecutive segments that will trigger the detection rule. The drop in size often indicates an end of request or response. A value of ”0” means unlimited. with a non-zero minimum of ”2”. The service names can be any of those used in the host attribute table (see 2.3) or others speciﬁc to the network. that is the default TCP policy. The default value is ”0” (disabled). Useful in ips mode to ﬂush upon seeing a drop in segment size after N segments of non-decreasing size. Specify the client. preprocessor stream5_udp: [timeout <number secs>].7. This is a performance improvement and may result in missed attacks.
Stream5 UDP Conﬁguration Conﬁguration for UDP session tracking.

conf and can be used for repeatable tests of stream reassembly in readback mode. A list of rule SIDs affected by this option are printed at Snort’s startup. a UDP rule will be ignored except when there is another port speciﬁc rule With the ignore
that may be applied to the trafﬁc. use_static_footprint_sizes preprocessor stream5_udp: \ ignore_any_rules 2. This example conﬁguration is the default conﬁguration in snort. the minimum is ”1”. with all other trafﬁc going to the default policy of Solaris. preprocessor stream5_icmp: [timeout <number secs>] Option timeout <num seconds> Description Session timeout.
! △NOTE untested. It is not ICMP is currently
turned on by default. the minimum is ”1”. track_udp yes. The default is ”30”. if a UDP rule speciﬁes destination port 53. Don’t process any -> any (ports) rules for UDP that attempt to match payload if there are no port speciﬁc rules for the src or destination port. but NOT to any other source or destination port.
! △NOTE any rules option. PCRE. For example. The default is ”30”. there should be only one occurrence of the ICMP conﬁguration.
! △NOTE any rules option. This conﬁguration maps two network segments to different OS policies. or byte test options. the ignore any rules option will be disabled in this case. 45
.Option timeout <num seconds> ignore any rules
Description Session timeout. and the maximum is ”86400” (approximately 1 day). and the maximum is ”86400” (approximately 1 day). Because of the potential impact of disabling a ﬂowbits rule. track_tcp yes. track_icmp no preprocessor stream5_tcp: \ policy first. This is a performance improvement and may result in missed attacks. only those with content. in minimal code form and is NOT ready for use in production networks. Since there is no target based binding. the ’ignored’ any -> any rule will be applied to trafﬁc to/from port 53. if a UDP rule that uses any -> any ports includes either ﬂow or ﬂowbits. Stream5 ICMP Conﬁguration Conﬁguration for ICMP session tracking. Using this does not affect rules that look at protocol headers. Rules that have ﬂow or ﬂowbits will never be ignored. With the ignore
the ignore any rules option is effectively pointless. preprocessor stream5_global: \ max_tcp 8192. The default is ”off”. one for Windows and one for Linux.
Example Conﬁgurations 1.

sfPortscan will currently alert for the following types of Nmap scans: • TCP Portscan • UDP Portscan • IP Portscan These alerts are for one→one portscans. is designed to detect the ﬁrst phase in a network attack: Reconnaissance. In the Reconnaissance phase.0/24.1. an attacker determines what types of network protocols or services a host supports. As the attacker has no beforehand knowledge of its intended target. this phase would not be necessary. This tactic helps hide the true identity of the attacker. so we track this type of scan through the scanned host. This is used to evade an IDS and obfuscate command and control hosts. sfPortscan also alerts for the following types of decoy portscans: • TCP Decoy Portscan • UDP Decoy Portscan • IP Decoy Portscan Decoy portscans are much like the Nmap portscans described above.168. and rarer still are multiple negative responses within a given amount of time. sfPortscan alerts for the following types of distributed portscans: • TCP Distributed Portscan • UDP Distributed Portscan • IP Distributed Portscan These are many→one portscans.1. Most of the port queries will be negative. This phase assumes the attacking host has no prior knowledge of what protocols or services are supported by the target. if not all. most queries sent by the attacker will be negative (meaning that the service ports are closed).preprocessor preprocessor preprocessor preprocessor
stream5_global: track_tcp yes stream5_tcp: bind_to 192. developed by Sourceﬁre. Nmap encompasses many.2. Distributed portscans occur when multiple hosts query one host for open services. otherwise. Our primary objective in detecting portscans is to detect and track these negative responses. policy windows stream5_tcp: bind_to 10. since most hosts have relatively few services available. negative responses from hosts are rare.1.3 sfPortscan
The sfPortscan module. In the nature of legitimate network communications. one host scans multiple ports on another host. This is the traditional place where a portscan takes place. policy linux stream5_tcp: policy solaris
2. One of the most common portscanning tools in use today is Nmap. only the attacker has a spoofed source address inter-mixed with the real scanning address.0/24.
! △NOTE
Negative queries will be distributed among scanning hosts. of the current portscanning techniques.
sfPortscan alerts for the following types of portsweeps: 46
. sfPortscan was designed to be able to detect the different types of scans Nmap can produce. which are the traditional types of scans.

if an attacker portsweeps a web farm for port 80. This usually occurs when a new exploit comes out and the attacker is looking for a speciﬁc service. Active hosts.• TCP Portsweep • UDP Portsweep • IP Portsweep • ICMP Portsweep These alerts are for one→many portsweeps. we will most likely not see many negative responses.
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. On TCP sweep alerts however. sfPortscan will only track open ports after the alert has been triggered. On TCP scan alerts.
sfPortscan alerts on the following ﬁltered portscans and portsweeps: • TCP Filtered Portscan • UDP Filtered Portscan • IP Filtered Portscan • TCP Filtered Decoy Portscan • UDP Filtered Decoy Portscan • IP Filtered Decoy Portscan • TCP Filtered Portsweep • UDP Filtered Portsweep • IP Filtered Portsweep • ICMP Filtered Portsweep • TCP Filtered Distributed Portscan • UDP Filtered Distributed Portscan • IP Filtered Distributed Portscan “Filtered” alerts indicate that there were no network errors (ICMP unreachables or TCP RSTs) or responses on closed ports have been suppressed. can trigger these alerts because they can send out many connection attempts within a very small amount of time. For example. One host scans a single port on multiple hosts. A ﬁltered alert may go off before responses from the remote hosts are received. but tags based on the original scan alert. It’s also a good indicator of whether the alert is just a very active legitimate host. Open port events are not individual alerts.
! △NOTE
The characteristics of a portsweep scan may not result in many negative responses. such as NATs. sfPortscan only generates one alert for each host pair in question during the time window (more on windows below). sfPortscan will also display any open ports that were scanned.

and speciﬁc ports on those hosts to watch. this ﬁle will be placed in the Snort conﬁg dir. A ”High” setting will catch some slow scans because of the continuous monitoring. as described in Section 2. This setting may false positive on active hosts (NATs. 5. ignore scanned <ip1|ip2/cidr[ [port|port2-port3]]> Ignores the destination of scan alerts. sense level <level> Available options: • low . 48
.2.“High” alerts continuously track hosts on a network using a time window to evaluate portscan statistics for that host.2. • high .“Low” alerts are only generated on error packets sent from the target host. IP address using CIDR notation. logﬁle <ﬁle> This option will output portscan events to the ﬁle speciﬁed. The parameter is the same format as that of watch ip. However. and so will generate ﬁltered scan alerts. 6. but is very sensitive to active hosts. and because of the nature of error responses. This most deﬁnitely will require the user to tune sfPortscan. 4. Stream gives portscan direction in the case of connectionless protocols like ICMP and UDP. this setting should see very few false positives.conf. • medium . ignore scanners <ip1|ip2/cidr[ [port|port2-port3]]> Ignores the source of scan alerts. ports are speciﬁed after the IP address/CIDR using a space and can be either a single port or a range denoted by a dash. The parameter is the same format as that of watch ip. etc). DNS caches. so the user may need to deploy the use of Ignore directives to properly tune this directive. scan type <scan type> Available options: • portscan • portsweep • decoy portscan • distributed portscan • all 3. 7. IPs or networks not falling into this range are ignored if this option is used. If file does not contain a leading slash. The parameters you can use to conﬁgure the portscan module are: 1.“Medium” alerts track connection counts. You should enable the Stream preprocessor in your snort. proto <protocol> Available options: • TCP • UDP • IGMP • ip proto • all 2. The list is a comma separated list of IP addresses. this setting will never trigger a Filtered Scan alert because of a lack of error responses. networks. after which this window is reset. Optionally. proxies. watch ip <ip1|ip2/cidr[ [port|port2-port3]]> Deﬁnes which IPs.sfPortscan Conﬁguration Use of the Stream5 preprocessor is required for sfPortscan. This setting is based on a static time window of 60 seconds.

port count. then the user won’t see open port alerts. 10. especially under heavy load with dropped packets. the packet looks like the IP portion of the packet that caused the portscan alert to be generated. However. The payload and payload size of the packet are equal to the length of the additional portscan information that is logged. snort generates a pseudo-packet and uses the payload portion to store the additional portscan information of priority count. The other options are parsed but not used.
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. because open port alerts utilize the tagged packet output system. The characteristics of the packet are: Src/Dst MAC Addr == MACDAD IP Protocol == 255 IP TTL == 0 Other than that. IP range. which is why the option is off by default. Open port alerts differ from the other portscan alerts. and port range. Any valid conﬁguration may have ”disabled” added to it. so it is possible to extend favorite Snort GUIs to display portscan alerts and the additional information in the IP payload using the above packet characteristics. This can lead to false alerts. especially under heavy load with dropped packets. The open port information is stored in the IP payload and contains the port that is open. detect ack scans This option will include sessions picked up in midstream by the stream module. this can lead to false alerts. This means that if an output system that doesn’t print tagged packets is used. include midstream This option will include sessions picked up in midstream by Stream5. This option disables the preprocessor.200 bytes. The size tends to be around 100 . which is why the option is off by default. This includes any IP options. which is necessary to detect ACK scans. etc. disabled This optional keyword is allowed with any policy to avoid packet processing. IP count. When the preprocessor is disabled only the memcap option is applied when speciﬁed with the conﬁguration. The sfPortscan alert output was designed to work with uniﬁed packet logging.8. Format preprocessor sfportscan: proto <protocols> \ scan_type <portscan|portsweep|decoy_portscan|distributed_portscan|all> \ sense_level <low|medium|high> \ watch_ip <IP or IP/CIDR> \ ignore_scanners <IP list> \ ignore_scanned <IP list> \ logfile <path and filename> \ disabled Example preprocessor flow: stats_interval 0 hash 2 preprocessor sfportscan:\ proto { all } \ scan_type { all } \ sense_level { low } sfPortscan Alert Output Uniﬁed Output In order to get all the portscan information logged with the alert. connection count. 9.

5 (portscan) TCP Filtered Portscan Priority Count: 0 Connection Count: 200 IP Count: 2 Scanner IP Range: 192. The watch ip option is easy to understand. this is a low number.168. 3. and explained further below: Time: 09/08-15:07:31. We use this count (along with IP Count) to determine the difference between one-to-one portscans and one-to-one decoys.603880 event_id: 2 192. the more bad responses have been received.169. 6.169.3 -> 192. Portsweep (one-to-many) scans display the scanned IP range. Tuning sfPortscan The most important aspect in detecting portscans is tuning the detection engine for your network(s).3 -> 192. sfPortscan will watch all network trafﬁc. and one-to-one scans may appear as a distributed scan.169.4 Port/Proto Count: 200 Port/Proto Range: 20:47557 If there are open ports on the target. The analyst should set this option to the list of CIDR blocks and IPs that they want to watch. and ignore scanned options.
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. 5.Log File Output Log ﬁle output is displayed in the following format. Connection Count Connection Count lists how many connections are active on the hosts (src or dst).3:192. The higher the priority count. If no watch ip is deﬁned. Here are some tuning tips: 1.169. For active hosts this number will be high regardless. 4. Use the watch ip. and is more of an estimate for others. unreachables).168. Event id/Event ref These ﬁelds are used to link an alert with the corresponding Open Port tagged packet 2. Scanned/Scanner IP Range This ﬁeld changes depending on the type of alert.5 (portscan) Open Port Open Port: 38458 1.169.168.168. It’s important to correctly set these options. Whether or not a portscan was ﬁltered is determined here. For one-to-one scans. IP Count IP Count keeps track of the last IP to contact a host. Portscans (one-to-one) display the scanner IP. ignore scanners. and increments the count if the next IP is different.603881 event_ref: 2 192. This is accurate for connection-based protocols. one or more additional tagged packet(s) will be appended: Time: 09/08-15:07:31.169. Priority Count Priority Count keeps track of bad responses (resets. Port Count Port Count keeps track of the last port contacted and increments this number when that changes.168. High connection count and low priority count would indicate ﬁltered (no response received from target).168.

If stream5 is enabled. This indicates that each connection was to a different port. By default. but be aware when ﬁrst tuning sfPortscan for these IPs. Connection Count. For portscans. The portscan alert details are vital in determining the scope of a portscan and also the conﬁdence of the portscan. the analyst will know which to ignore it as. then add it to the ignore scanners option. For portscans.2. If the host is generating portsweep events. Many times this just indicates that a host was very active during the time period in question. When determining false positives. add it to the ignore scanned option. You get the best protection the higher the sensitivity level. The low sensitivity level does not catch ﬁltered scans. and nfs servers. IP Range. This indicates that there were many connections to the same port. and Port Range to determine false positives. Some of the most common examples are NAT IPs. this ratio should be low. The easiest way to determine false positives is through simple ratio estimations. The following is a list of ratios to estimate and the associated values that indicate a legitimate scan and not a false positive. this ratio should be low. For portsweeps. Depending on the type of alert that the host generates. but it’s also important that the portscan detection engine generate alerts that the analyst will ﬁnd informative. lower the sensitivity level. 3. For portsweeps. The reason that Priority Count is not included. So be much more suspicious of ﬁltered portscans. Port Count. syslog servers. the higher the better. 4. sfPortscan may not generate false positives for these types of hosts. it will only process client-side trafﬁc. These responses indicate a portscan and the alerts generated by the low sensitivity level are highly accurate and require the least tuning. Connection Count / IP Count: This ratio indicates an estimated average of connections per IP. Most of the false positives that sfPortscan may generate are of the ﬁltered scan alert type. DNS cache servers. the alert type is very important. For portscans. Connection Count / Port Count: This ratio indicates an estimated average of connections per port. Make use of the Priority Count. Port Count / IP Count: This ratio indicates an estimated average of ports connected to per IP. we hope to automate much of this analysis in assigning a scope level and conﬁdence level. Filtered scan alerts are much more prone to false positives. If all else fails. IP Count. If the host continually generates these types of alerts. it runs against trafﬁc on ports 111 and 32771. but for now the user must manually do this.4 RPC Decode
The rpc decode preprocessor normalizes RPC multiple fragmented records into a single un-fragmented record. this ratio should be high.The ignore scanners and ignore scanned options come into play in weeding out legitimate hosts that are very active on your network. this ratio should be high. this ratio should be low. indicating that the scanning host connected to few ports but on many hosts. The Priority Count play an important role in tuning because the higher the priority count the more likely it is a real portscan or portsweep (unless the host is ﬁrewalled). In the future. If none of these other tuning techniques work or the analyst doesn’t have the time for tuning. If the host is generating portscan alerts (and is the host that is being scanned). 2. It does this by normalizing the packet into the packet buffer. this ratio should be high and indicates that the scanned host’s ports were connected to by fewer IPs. is because the priority count is included in the connection count and the above comparisons take that into consideration. Format preprocessor rpc_decode: \ <ports> [ alert_fragments ] \ [no_alert_multiple_requests] \ [no_alert_large_fragments] \ [no_alert_incomplete]
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. lower the sensitivity level.
2. The low sensitivity level only generates alerts based on error responses. since these are more prone to false positives. add it to the ignore scanners list or use a lower sensitivity level. For portsweeps.

since checking the time sample reduces Snort’s performance. • accumulate or reset . Rules with short. so if possible.Prints out statistics about the type of trafﬁc and protocol distributions that Snort is seeing.• Cached UDP Ssns Del/Sec • Current Cached UDP Sessions • Max Cached UDP Sessions • Current Attribute Table Hosts (Target Based) • Attribute Table Reloads (Target Based) • Mbits/Sec (Snort) • Mbits/Sec (snifﬁng) • Mbits/Sec (combined) • uSeconds/Pkt (Snort) • uSeconds/Pkt (snifﬁng) • uSeconds/Pkt (combined) • KPkts/Sec (Snort) • KPkts/Sec (snifﬁng) • KPkts/Sec (combined) The following options can be used with the performance monitor: • flow . • atexitonly .Dump stats for entire life of Snort. By default. Not all statistics are output to this ﬁle. This boosts performance. adding a content rule option to those rules can decrease the number of times they need to be evaluated and improve performance. This option can produce large amounts of output. • max . • console . • file . You may also use snortfile which will output into your deﬁned Snort log directory.Adjusts the number of packets to process before checking for the time sample. This prints out statistics as to the number of rules that were evaluated and didn’t match (non-qualiﬁed events) vs. • events . • time . A high non-qualiﬁed event to qualiﬁed event ratio can indicate there are many rules with either minimal content or no content that are being evaluated without success. This is only valid for uniprocessor machines.Prints statistics at the console. since many operating systems don’t keep accurate kernel statistics for multiple CPUs. this is 10000.Represents the number of seconds between intervals. Rules without content are not ﬁltered via the fast pattern matcher and are always evaluated. 54
. generic contents are more likely to be selected for evaluation than those with longer. reset is used.Turns on event reporting. the number of rules that were evaluated and matched (qualiﬁed events).Deﬁnes which type of drop statistics are kept by the operating system. By default. At startup. more unique contents.Prints statistics in a comma-delimited format to the ﬁle that is speciﬁed.Turns on the theoretical maximum performance that Snort calculates given the processor speed and current performance. The fast pattern matcher is used to select a set of rules for evaluation based on the longest content or a content modiﬁed with the fast pattern rule option in a rule. Both of these directives can be overridden on the command line with the -Z or --perfmon-file options. • pktcnt . Snort will log a distinctive line to this ﬁle with a timestamp to all readers to easily identify gaps in the stats caused by Snort not running.

2. The following example gives the generic global conﬁguration format: 55
.Collects IP trafﬁc distribution statistics based on host pairs. This means that HTTP Inspect looks for HTTP ﬁelds on a packet-by-packet basis. • flow-ip-file . the table will start to prune the statistics for the least recently seen host pairs to free memory. • flow-ip . followed by YYYY-MM-DD. All of the statistics mentioned above. This works ﬁne when there is another module handling the reassembly. This value is in bytes and the default value is 52428800 (50MB). it will be rolled into a new date stamped ﬁle of the format YYYY-MM-DD. ﬁnd HTTP ﬁelds.Deﬁnes the maximum size of the comma-delimited ﬁle. where x will be incremented each time the comma delimited ﬁle is rolled over. Global Conﬁguration The global conﬁguration deals with conﬁguration options that determine the global functioning of HTTP Inspect. and will be fooled if packets are not reassembled.x. which should allow the user to emulate any type of web server. • flow-ip-memcap . but there are limitations in analyzing the protocol. For each pair of hosts for which IP trafﬁc has been seen. The minimum is 4096 bytes and the maximum is 2147483648 bytes (2GB). as well as the IP addresses of the host pairs in human-readable format. Users can conﬁgure individual HTTP servers with a variety of options. Before the ﬁle exceeds this size. and normalize the ﬁelds. Once the cap has been reached. HTTP Inspect has a very “rich” user conﬁguration. HTTP Inspect works on both client requests and server responses.csv pktcnt 1000
2.Sets the memory cap on the hash table used to store IP trafﬁc statistics for host pairs. HTTP Inspect will decode the buffer.• max file size . the following statistics are collected for both directions (A to B and B to A): – TCP Packets – TCP Trafﬁc in Bytes – TCP Sessions Established – TCP Sessions Closed – UDP Packets – UDP Trafﬁc in Bytes – UDP Sessions Created – Other IP Packets – Other IP Trafﬁc in Bytes These statistics are printed and reset at the end of each interval. Examples preprocessor perfmonitor: \ time 30 events flow file stats. The current version of HTTP Inspect only handles stateless processing. there are two areas of conﬁguration: global and server. Within HTTP Inspect.Prints the ﬂow IP statistics in a comma-delimited format to the ﬁle that is speciﬁed. The default is the same as the maximum. are included.profile max console pktcnt 10000 preprocessor perfmonitor: \ time 300 file /var/tmp/snortstat pktcnt 10000 preprocessor perfmonitor: \ time 30 flow-ip flow-ip-file flow-ip-stats. Given a data buffer. Future versions will have a stateful processing mode which will hook into various reassembly modules.6 HTTP Inspect
HTTP Inspect is a generic HTTP decoder for user applications.

you will only receive proxy use alerts for web users that aren’t using the conﬁgured proxies or are using a rogue proxy server. The map ﬁle can reside in the same directory as snort. Don’t turn this on if you don’t have a default server conﬁguration that encompasses all of the HTTP server ports that your users might access. which is available at http://www. The iis unicode map is a required conﬁguration parameter. 2. please only use this feature with traditional proxy environments. we want to limit this to speciﬁc networks so it’s more useful. but for right now. This option is turned off by default. in case of multiple policies.Format preprocessor http_inspect: \ global \ iis_unicode_map <map_filename> \ codemap <integer> \ [detect_anomalous_servers] \ [proxy_alert] \ [max_gzip_mem <num>] \ [compress_depth <num>] [decompress_depth <num>] \ disabled You can only have a single global conﬁguration.map and should be used if no other codepoint map is available. and alerts if HTTP trafﬁc is seen.
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. This value can be set from 1 to 65535. Conﬁguration 1. The iis unicode map ﬁle is a Unicode codepoint map which tells HTTP Inspect which codepage to use when decoding Unicode characters. A tool is supplied with Snort to generate custom Unicode maps--ms unicode generator.conf or be speciﬁed via a fully-qualiﬁed path to the map ﬁle. A Microsoft US Unicode codepoint map is provided in the Snort source etc directory by default. The default for this option is 1460. proxy alert This enables global alerting on HTTP server proxy usage. this inspects all network trafﬁc. It is called unicode. iis unicode map <map filename> [codemap <integer>] This is the global iis unicode map ﬁle.
! △NOTE
Remember that this conﬁguration is for the global IIS Unicode map.snort. Blind ﬁrewall proxies don’t count. This value should be speciﬁed in the default policy even when the HTTP inspect is turned off using the disabled keyword.org/dl/contrib/. So. In case of unlimited decompress this should be set to its max value. you’ll get an error if you try otherwise.c. individual servers can reference their own IIS Unicode map. the value speciﬁed in the default policy is used and this value overwrites the values speciﬁed in the other policies. the codemap is usually 1252. detect anomalous servers This global conﬁguration option enables generic HTTP server trafﬁc inspection on non-HTTP conﬁgured ports. By conﬁguring HTTP Inspect servers and enabling allow proxy use. For US servers. Please note that if users aren’t required to conﬁgure web proxy use. 3. In the future. then you may get a lot of proxy alerts.
! △NOTE
Please note. compress depth <integer> This option speciﬁes the maximum amount of packet payload to decompress. 4.

When the preprocessor is disabled only the ”max gzip mem”. 6. Example Default Conﬁguration preprocessor http_inspect_server: \ server default profile all ports { 80 } Conﬁguration by IP Address This format is very similar to “default”. max gzip mem This option determines (in bytes) the maximum amount of memory the HTTP Inspect preprocessor will use for decompression. Example Global Conﬁguration preprocessor http_inspect: \ global iis_unicode_map unicode. This value can be set from 1 to 65535.
! △NOTE
Please note. ”compress depth” and ”decompress depth” options are applied when speciﬁed with the conﬁguration. the only difference being that speciﬁc IPs can be conﬁgured. disabled This optional keyword is allowed with any policy to avoid packet processing.
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. Other options are parsed but not used. Default This conﬁguration supplies the default server conﬁguration for any server that is not individually conﬁgured.map 1252 Server Conﬁguration There are two types of server conﬁgurations: default and by IP address. The default value for this option is 838860. max gzip session = max gzip mem /(decompress depth + compress depth) 7. This value can be set from 3276 bytes to 100MB.
! △NOTE
This value should be speciﬁed in the default policy even when the HTTP inspect is turned off using the disabled keyword. In case of unlimited decompress this should be set to its max value. in case of multiple policies. This option disables the preprocessor. This value should be speciﬁed in the default policy even when the HTTP inspect is turned off using the disabled keyword. It is suggested to set this value such that the max gzip session calculated as follows is at least 1. decompress depth <integer> This option speciﬁes the maximum amount of decompressed data to obtain from the compressed packet payload.5. the value speciﬁed in the default policy is used and this value overwrites the values speciﬁed in the other policies. Any valid conﬁguration may have ”disabled” added to it. The default for this option is 2920. Most of your web servers will most likely end up using the default conﬁguration. This option along with compress depth and decompress depth determines the gzip sessions that will be decompressed at any given instant.

We alert on the more serious forms of evasions. Apache also accepts tabs as whitespace. iis5 0 In IIS 4. profile apache sets the conﬁguration options described in Table 2.1 profile all ports { 80 } Conﬁguration by Multiple IP Addresses This format is very similar to “Conﬁguration by IP Address”. profile all sets the conﬁguration options described in Table 2. backslashes.5. double decoding. bare-byte encoding. There are ﬁve proﬁles available: all.1.6. This argument speciﬁes whether the user wants the conﬁguration option to generate an HTTP Inspect alert or not. 1-B. except they will alert by default if a URL has a double encoding.Example IP Conﬁguration preprocessor http_inspect_server: \ server 10. iis4 0. profile <all|apache|iis|iis5 0|iis4 0> Users can conﬁgure HTTP Inspect by using pre-deﬁned HTTP server proﬁles.0 and IIS 5. etc.0/24 } profile all ports { 80 } Server Conﬁguration Options Important: Some conﬁguration options have an argument of ‘yes’ or ‘no’. These two proﬁles are identical to iis. This is a great proﬁle for detecting all types of attacks. Double decode is not supported in IIS 5. The ‘yes/no’ argument does not specify whether the conﬁguration option itself is on or off.1. regardless of the HTTP server. 1.2. default. apache. Proﬁles must be speciﬁed as the ﬁrst server option and cannot be combined with any other options except: • • • • • ports iis unicode allow proxy server flow client flow map use depth depth 58
1-E. whether set to ‘yes’ or ’no’. Proﬁles allow the user to easily conﬁgure the preprocessor for a certain type of server. all The all proﬁle is meant to normalize the URI using most of the common tricks available. iis. %u encoding. iis5 0. only the alerting functionality. apache The apache proﬁle is used for Apache web servers.1 10.2. iis The iis proﬁle mimics IIS servers.3. This differs from the iis proﬁle by only accepting UTF-8 standard Unicode encoding and not accepting backslashes as legitimate slashes.1. so it’s disabled by default. There is a limit of 40 IP addresses or CIDR notations per http inspect server line. but are not required for proper operation.4.
. So that means we use IIS Unicode codemaps for each server. like IIS does. 1-A. 1-D.
1-C. and rules based on HTTP trafﬁc will still trigger. HTTP normalization will still occur. Example Multiple IP Conﬁguration preprocessor http_inspect_server: \ server { 10.1. In other words. there was a double decoding vulnerability. the only difference being that multiple IPs can be speciﬁed via a space separated list.1 and beyond.0. no profile The default options used by HTTP Inspect do not use a proﬁle and are described in Table 2. and iis4 0. profile iis sets the conﬁguration options described in Table 2.

You should select the conﬁg option ”extended response inspection” before conﬁguring this option. To search for patterns in the header of the response. By turning this option the HTTP response will be thoroughly inspected. Different rule options are provided to inspect these buffers. the decompressed data from different packets are not combined while inspecting). When using this option. alert off non strict url parsing on tab uri delimiter is set max header length 0. http stat msg and http cookie.Table 2.
! △NOTE
To enable compression of HTTP server response. extended response inspection This enables the extended HTTP response inspection. The different ﬁelds of a HTTP response such as status code. this is usually 1252. alert on apache whitespace on.
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. alert off webroot on. cookie (when enable cookie is conﬁgured) and body are extracted and saved into buffers. So the decompression will end when either the ’compress depth’ or ’decompress depth’ is reached or when the compressed data ends. Snort should be conﬁgured with the –enable-zlib ﬂag. The Cookie header line is extracted and stored in HTTP Cookie buffer for HTTP requests and Set-Cookie is extracted and stored in HTTP Cookie buffer for HTTP responses. Also the amount of decompressed data that will be inspected depends on the ’server ﬂow depth’ conﬁgured. one should use the http modiﬁers with content such as http header. if the HTTP response packet has a body then any content pattern matches ( without http modiﬁers ) will search the response body ((decompressed in case of gzip) and not the entire packet payload. But the decompressed data are individually inspected. When the compressed data is spanned across multiple packets. status message.e. You can select the correct code page by looking at the available code pages that the ms unicode generator outputs. The default http response inspection does not inspect the various ﬁelds of a HTTP response.
! △NOTE
When this option is turned on. inspect gzip This option speciﬁes the HTTP inspect module to uncompress the compressed data(gzip/deﬂate) in HTTP response. So. the state of the last decompressed packet is used to decompressed the data of the next packet. But the ms unicode generator program tells you which codemap to use for you server. alert off directory normalization on. number of headers not checked
Unicode map for the system that it was run on. alert off multiple slash on. 6. By default the cookie inspection and extraction will be turned off. alert on utf 8 encoding on. 4. the user needs to specify the ﬁle that contains the IIS Unicode map and also specify the Unicode map to use.4: Options for the apache Proﬁle Option Setting server ﬂow depth 300 client ﬂow depth 300 post depth 0 chunk encoding alert on chunks larger than 500000 bytes ASCII decoding on. you run this program on that server and use that Unicode map in this conﬁguration. http stat code. In both cases the header name is also stored along with the cookie. 5. it’s the ANSI code page. enable cookie This options turns on the cookie extraction from HTTP requests and HTTP response. Decompression is done across packets. headers. (i. For US servers. header length not checked max headers 0. to get the speciﬁc Unicode mappings for an IIS web server.

or the content that is likely to be in the ﬁrst hundred or so bytes of non-header data. alert on iis backslash on. alert off directory normalization on. Values above 0 tell Snort the number of bytes to inspect of the server response
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. When extended response inspection is turned on. it is applied to the HTTP response body (decompressed data when inspect gzip is turned on) and not the HTTP headers. alert on iis unicode codepoints on. This option can be used to balance the needs of IDS performance and level of inspection of HTTP server response data. alert on double decoding on. but your mileage may vary. When the extended response inspection is turned on. alert off webroot on. This value can be set from -1 to 65535. Headers are usually under 300 bytes long. It is suggested to set the server flow depth to its maximum value. The decompression in a single packet is still limited by the ’compress depth’ and ’decompress depth’. Most of these rules target either the HTTP header.Decompression will stop when the compressed data ends or when a out of sequence packet is received. 9. Inversely. value of -1 causes Snort to ignore the HTTP response body data and not the HTTP headers. A value of -1 causes Snort to ignore all server side trafﬁc for ports deﬁned in ports when extended response inspection is turned off. alert on %u decoding on. unlimited decompress This option enables the user to decompress unlimited gzip data (across multiple packets). header length not checked max headers 0. enable xff This option enables Snort to parse and log the original client IP present in the X-Forwarded-For or True-ClientIP HTTP request headers along with the generated events. server flow depth <integer> This speciﬁes the amount of server response payload to inspect. The XFF/True-Client-IP Original client IP address is logged only with uniﬁed2 output and is not logged with console (-A cmg) output. user should set the ’compress depth’ and ’decompress depth’ to its maximum values in the default policy. number of headers not checked
7. alert off iis delimiter on.5: Options for the iis Proﬁle Option Setting server ﬂow depth 300 client ﬂow depth 300 post depth -1 chunk encoding alert on chunks larger than 500000 bytes iis unicode map codepoint map in the global conﬁguration ASCII decoding on. 8.
! △NOTE
The original client IP from XFF/True-Client-IP in uniﬁed2 logs can be viewed using the tool u2spewfoo. alert off multiple slash on. Unlike client flow depth this option is applied per TCP session. This tool is present in the tools/u2spewfoo directory of snort source tree. Snort rules are targeted at HTTP server response trafﬁc and when used with a small ﬂow depth value may cause false negatives. To ensure unlimited decompression. alert on bare byte decoding on.Table 2. a value of 0 causes Snort to inspect all HTTP server payloads deﬁned in ”ports” (note that this will likely slow down IDS performance). alert on non strict URL parsing on max header length 0. When extended response inspection is turned off the server flow depth is applied to the entire HTTP response (including headers). alert on apache whitespace on.

If less than ﬂow depth bytes are in the payload of the HTTP response packets in a given session. The value can be set from -1 to 65495. alert off multiple slash on. Only packets payloads starting with ’HTTP’ will be considered as the ﬁrst packet of a server response. client flow depth <integer> This speciﬁes the amount of raw client request payload to inspect. 11. It is suggested to set the server flow depth to its maximum value. It has a default value of 300. Values above 0 tell Snort the number of bytes to inspect in the ﬁrst packet of the client request. Inversely. This increases the performance by inspecting only speciﬁed bytes in the post message. A value of -1 causes Snort to ignore all the data in the post message. post depth <integer> This speciﬁes the amount of data to inspect in a client post message. alert off directory normalization on. alert on iis backslash on. Rules that are meant to inspect data in the payload of the HTTP response packets in a session beyond 65535 bytes will be ineffective unless ﬂow depth is set to 0. alert off non strict URL parsing on max header length 0. alert off utf 8 encoding on. number of headers not checked
(excluding the HTTP headers when extended response inspection is turned on) in a given HTTP session. alert off apache whitespace on. Note that the 65535 byte maximum ﬂow depth applies to stream reassembled packets as well.6: Default HTTP Inspect Options Option Setting port 80 server ﬂow depth 300 client ﬂow depth 300 post depth -1 chunk encoding alert on chunks larger than 500000 bytes ASCII decoding on. The default value for server flow depth is 300.” Inversely. a value of 0 causes Snort to inspect all HTTP client side trafﬁc deﬁned in ”ports” (note that this will likely slow down IDS performance).
! △NOTE
server flow depth is the same as the old flow depth option. the entire payload will be inspected. This value can be set from -1 to 1460. The default value is -1. which will be deprecated in a future release. If more than ﬂow depth bytes are in the payload of the HTTP response packet in a session only ﬂow depth bytes of the payload will be inspected for that session. Rules that are meant to inspect data in the payload of the ﬁrst packet of a client request beyond 1460 bytes will be ineffective unless ﬂow depth is set to 0.
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. If more than ﬂow depth bytes are in the payload of the ﬁrst packet only ﬂow depth bytes of the payload will be inspected. Note that the 1460 byte maximum ﬂow depth applies to stream reassembled packets as well. the entire payload will be inspected.Table 2. It is not a session based ﬂow depth. It is suggested to set the client flow depth to its maximum value. a value of 0 causes Snort to inspect all the client post message. It primarily eliminates Snort from inspecting larger HTTP Cookies that appear at the end of many client request Headers. A value of -1 causes Snort to ignore all client side trafﬁc for ports deﬁned in ”ports. header length not checked max headers 0. Unlike server flow depth this value is applied to the ﬁrst packet of the HTTP request. If less than ﬂow depth bytes are in the TCP payload (HTTP request) of the ﬁrst packet. alert off webroot on. alert off iis delimiter on. 10.

bare byte <yes|no> Bare byte encoding is an IIS trick that uses non-ASCII characters as valid values when decoding UTF-8 values.yk. the following encodings are done: ASCII. In the ﬁrst pass. When utf 8 is enabled.
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. 14. The xxxx is a hex-encoded value that correlates to an IIS Unicode codepoint. 18. iis unicode <yes|no> The iis unicode option turns on the Unicode codepoint mapping. In the second pass. How this works is that IIS does two passes through the request URI. ASCII and UTF-8 decoding are also enabled to enforce correct decoding. 13.a %2f = /. this option will not work. An ASCII character is encoded like %u002f = /..k. because it is seen mainly in attacks and evasion attempts. double decode <yes|no> The double decode option is once again IIS-speciﬁc and emulates IIS functionality. Don’t use the %u option.12. it seems that all types of iis encoding is done: utf-8 unicode. base36 <yes|no> This is an option to decode base36 encoded chars. This value can most deﬁnitely be ASCII. When base36 is enabled. Anyway. you may be interested in knowing when you have a UTF-8 encoded URI. %2e = . so ASCII is also enabled to enforce correct decoding.jp/˜shikap/patch/spp_http_decode. This option is turned off by default and is not supported with any of the proﬁles.. This is not in the HTTP standard. extended ascii uri This option enables the support for extended ASCII codes in the HTTP request URI. u encode <yes|no> This option emulates the IIS %u encoding scheme. You should alert on the iis unicode option. If no iis unicode map is speciﬁed before or after this option. ASCII decoding is also enabled to enforce correct functioning. As for alerting. because we are not aware of any legitimate clients that use this encoding. and then UTF-8 is decoded in the second stage. and %u. 17. so it is recommended that you disable HTTP Inspect alerting for this option. Bare byte encoding allows the user to emulate an IIS server and interpret non-standard encodings correctly. 15. The iis unicode option handles the mapping of non-ASCII codepoints that the IIS server accepts and decodes normal UTF-8 requests. How the %u encoding scheme works is as follows: the encoding scheme is started by a %u followed by 4 characters. ASCII. make sure you have this option turned on. It is normal to see ASCII encoding usage in URLs. If %u encoding is enabled. When iis unicode is enabled. like %uxxxx. You have to use the base36 option with the utf 8 option. because there are no legitimate clients that encode UTF-8 this way since it is non-standard. etc. bare byte. The alert on this decoding should be enabled. because base36 won’t work. iis unicode uses the default codemap. 16. If there is no iis unicode map option speciﬁed with the server conﬁg. the default codemap is used. utf 8 <yes|no> The utf-8 decode option tells HTTP Inspect to decode standard UTF-8 Unicode sequences that are in the URI. To alert on UTF-8 decoding. Apache uses this standard. bare byte.patch. This option is based on info from: http://www. a. %u002e = . you must enable also enable utf 8 yes. ascii <yes|no> The ascii decode option tells us whether to decode encoded ASCII chars. You should alert on %u encodings. etc.rim. this is really complex and adds tons of different encodings for one character. So it is most likely someone trying to be covert.or. We leave out utf-8 because I think how this works is that the % encoded utf-8 is decoded to the Unicode byte in the ﬁrst pass. 19. This abides by the Unicode standard and only uses % encoding. ASCII encoding is also enabled to enforce correct behavior. as all non-ASCII values have to be encoded with a %. and %u. When double decode is enabled. so for any Apache servers. but this will be prone to false positives as legitimate web clients use this type of encoding. doing decodes in each one.

a user may not want to see null bytes in the request URI and we can alert on that. because you could conﬁgure it to say. no pipeline req This option turns HTTP pipeline decoding off. but may also be false positive prone. but when this option is enabled. alert on all ‘/’ or something like that. By default. iis delimiter <yes|no> This started out being IIS-speciﬁc.. so if the emulated web server is Apache. Apache uses this. we always take this as standard since the most popular web servers accept it. Alerts on this option may be interesting. non rfc char {<byte> [<byte . But you can still get an alert on this option. so be careful./bar gets normalized to: /foo/bar The directory: /foo/. The directory: /foo/fake\_dir/. enable this option. For instance. otherwise. apache whitespace <yes|no> This option deals with the non-RFC standard of using tab for a space delimiter.>]} This option lets users receive an alert if certain non-RFC chars are used in a request URI. then conﬁgure with a yes. so something like: “foo/////////bar” get normalized to “foo/bar. and is a performance enhancement if needed. specify yes. 25.” 23.20. 21. specify no. 24. pipeline requests are inspected for attacks. and may also alert on HTTP tunneling that uses chunk encoding.
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./bar gets normalized to: /foo/bar If you want to conﬁgure an alert. 26. multi slash <yes|no> This option normalizes multiple slashes in a row. otherwise. 27. This alert may give false positives. pipeline requests are not decoded and analyzed per HTTP protocol ﬁeld. use no. This is again an IIS emulation. directory <yes|no> This option normalizes directory traversals and self-referential directories. iis backslash <yes|no> Normalizes backslashes to slashes. This picks up the Apache chunk encoding exploits. Since this is common. since some web sites refer to ﬁles using directory traversals. chunk length <non-zero positive integer> This option is an anomaly detector for abnormally large chunk sizes.. but Apache takes this non-standard delimiter was well.” If you want an alert when multiple slashes are seen. It is only inspected with the generic pattern matching. 22.. It’s ﬂexible. So a request URI of “/foo\bar” gets normalized to “/foo/bar. Please use this option with care.

It’s important to note that if this option is used without any uricontent rules. enable this optimization. content: "foo". As this ﬁeld usually contains 90-95% of the web attacks. no alerts This option turns off all alerts that are generated by the HTTP Inspect preprocessor module. tab uri delimiter This option turns on the use of the tab character (0x09) as a delimiter for a URI. which is associated with certain web attacks. The non strict option assumes the URI is between the ﬁrst and second space even if there is no valid HTTP identiﬁer after the second space. Requests that exceed this length will cause a ”Long Header” alert.28. inspect uri only This is a performance optimization.html alsjdfk alsj lj aj la jsj s\n”. Specifying a value of 0 is treated as disabling the alert. No argument is speciﬁed. you’ll catch most of the attacks. No argument is speciﬁed. Apache accepts tab as a delimiter. This has no effect on HTTP rules in the rule set. then there is nothing to inspect. Only use this option on servers that will accept URIs like this: ”get /index. 34. 29. IIS does not.htm http/1. webroot <yes|no> This option generates an alert when a directory traversal traverses past the web server root directory. The integer is the maximum length allowed for an HTTP client request header ﬁeld. The inspect uri only conﬁguration turns off all forms of detection except uricontent inspection. If a url directory is larger than this argument size. oversize dir length <non-zero positive integer> This option takes a non-zero positive integer as an argument. The argument speciﬁes the max char directory length for URL directory. 32. then no inspection will take place. 33. 31. 35. 30. So if you need extra performance. This means that no alert will be generated if the proxy alert global keyword has been used. It only alerts when the directory traversals go past the web server root directory. When enabled. if we have the following rule set: alert tcp any any -> any 80 ( msg:"content". For IIS. If the proxy alert keyword is not enabled. because it doesn’t alert on directory traversals that stay within the web server directory structure. For example. To enable. non strict This option turns on non-strict URI parsing for the broken way in which Apache servers will decode a URI. then this option does nothing. A good argument value is 300 characters. This is obvious since the URI is only inspected with uricontent rules. This generates much fewer false positives than the directory option. max header length <positive integer up to 65535> This option takes an integer as an argument. This alert is off by default. allow proxy use By specifying this keyword. ) and the we inspect the following URI: get /foo. a tab is treated as whitespace if a space character (0x20) precedes it. and if there are none available. a tab in the URI should be treated as any other character.
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. The allow proxy use keyword is just a way to suppress unauthorized proxy use for an authorized server. Whether this option is on or not. like whisker -i 4.0\r\n\r\n No alert will be generated when inspect uri only is enabled. an alert is generated. the user is allowing proxy use on this server. only the URI portion of HTTP requests will be inspected for attacks. This should limit the alerts to IDS evasion type attacks. specify an integer argument to max header length of 1 to 65535.

37. 40. not including Cookies (using the same conﬁguration parameters as the URI normalization (ie.36. http methods {cmd[cmd]} This speciﬁes additional HTTP Request Methods outside of those checked by default within the preprocessor (GET and POST). The conﬁg option. etc. 39. max headers <positive integer up to 1024> This option takes an integer as an argument. Specifying a value of 0 is treated as disabling the alert.1 \ ports { 80 3128 8080 } \ server_flow_depth 0 \ ascii no \ double_decode yes \ non_rfc_char { 0x00 } \ chunk_length 500000 \ non_strict \ no_alerts preprocessor http_inspect_server: \ server default \ ports { 80 3128 } \ non_strict \ non_rfc_char { 0x00 } \ server_flow_depth 300 \ apache_whitespace yes \ directory no \ iis_backslash no \ u_encode yes \ 66
. normalize cookies This option turns on normalization for HTTP Cookie Fields (using the same conﬁguration parameters as the URI normalization (ie. To enable. specify an integer argument to max headers of 1 to 1024. tabs. normalize headers This option turns on normalization for HTTP Header Fields. normalize utf This option turns on normalization of HTTP response bodies where the Content-Type header lists the character set as ”utf-16le”. It is useful for normalizing Referrer URIs that may appear in the HTTP Header.). HTTP Inspect will attempt to normalize these back into 8-bit encoding. The alert is off by default. It is useful for normalizing data in HTTP Cookies that may be encoded.). multi-slash.1. ”utf-32le”. ”utf-16be”. 38. directory.1. http_methods { PUT CONNECT }
! △NOTE
Please note the maximum length for a method name is 7 Examples preprocessor http_inspect_server: \ server 10. or ”utf-32be”. The list should be enclosed within braces and delimited by spaces. The integer is the maximum number of HTTP client request header ﬁelds. multi-slash. etc. generating an alert if the extra bytes are non-zero. Requests that contain more HTTP Headers than this value will cause a ”Max Header” alert. braces and methods also needs to be separated by braces. directory. line feed or carriage return.

It will also mark the command. It saves state between individual packets. a loss of coherent stream data results in a loss of state). In addition. RFC 2821 recommends 512 as a maximum command line length. Since so few (none in the current snort rule set) exploits are against mail data. data header data body sections. 4. 3. Conﬁguration SMTP has the usual conﬁguration items. Typically. TLS-encrypted trafﬁc can be ignored. SMTP will decode the buffer and ﬁnd SMTP commands and responses. max command line len <int> Alert if an SMTP command line is longer than this value.. SMTP command lines can be normalized to remove extraneous spaces. ports { <port> [<port>] . Absence of this option or a ”0” means never alert on command line length. Space characters are deﬁned as space (ASCII 0x20) or tab (ASCII 0x09). which improves performance. 67
. cmds just checks commands listed with the normalize cmds parameter. inspection type <stateful | stateless> Indicate whether to operate in stateful or stateless mode. 2. SMTP handles stateless and stateful processing. and TLS data. 6. such as port and inspection type. all checks all commands none turns off normalization for all commands. regular mail data can be ignored for an additional performance boost.2. Normalization checks for more than one space character after a command.7 SMTP Preprocessor
The SMTP preprocessor is an SMTP decoder for user applications. normalize <all | none | cmds> This turns on normalization.ascii no \ chunk_length 500000 \ bare_byte yes \ double_decode yes \ iis_unicode yes \ iis_delimiter yes \ multi_slash no preprocessor http_inspect_server: \ server default \ profile all \ ports { 80 8080 }
2. Given a data buffer. The conﬁguration options are described below: 1. for encrypted SMTP. However maintaining correct state is dependent on the reassembly of the client side of the stream (ie.. this is relatively safe to do and can improve the performance of data inspection. ignore tls data Ignore TLS-encrypted data when processing rules. }
This speciﬁes on what ports to check for SMTP data. this will include 25 and possibly 465. Also. ignore data Ignore data section of mail (except for mail headers) when processing rules. 5.

Default is an empty list. enable mime decoding Enables Base64 decoding of Mime attachments/data. Default is an empty list. alert unknown cmds Alert if we don’t recognize command. Absence of this option or a ”0” means never alert on response line length. The default value for this in snort in 1460 bytes. 16.
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. Drop if alerted. When stateful inspection is turned on the base64 encoded MIME attachments/data across multiple packets are decoded too. See 3. invalid cmds { <Space-delimited list of commands> } Alert if this command is sent from client side. We do not alert on commands in this list. xlink2state { enable | disable [drop] } Enable/disable xlink2state alert.5. Default is enable. 8. 9. 17. This is useful when specifying the max mime depth and max mime mem in default policy without turning on the SMTP preprocessor. disabled Disables the SMTP preprocessor in a policy. 18. valid cmds { <Space-delimited list of commands> } List of valid commands. but preprocessor has this list hard-coded: { ATRN AUTH BDAT DATA DEBUG EHLO EMAL ESAM ESND ESOM ETRN EVFY EXPN } { HELO HELP IDENT MAIL NOOP QUIT RCPT RSET SAML SOML SEND ONEX QUEU } { STARTTLS TICK TIME TURN TURNME VERB VRFY X-EXPS X-LINK2STATE } { XADR XAUTH XCIR XEXCH50 XGEN XLICENSE XQUE XSTA XTRN XUSR } 13. Absence of this option or a ”0” means never alert on data header line length. The option take values ranging from 5 to 20480 bytes. no alerts Turn off all alerts for this preprocessor. max response line len <int> Alert if an SMTP response line is longer than this value. Default is off.7. RFC 2821 recommends 512 as a maximum response line length. 19. max mime depth <int> Speciﬁes the maximum number of base64 encoded data to decode per SMTP session. 11. normalize cmds { <Space-delimited list of commands> } Normalize this list of commands Default is { RCPT VRFY EXPN }. alt max command line len <int> { <cmd> [<cmd>] } Overrides max command line len for speciﬁc commands.24 rule option for more details. RFC 2821 recommends 1024 as a maximum data header line length. 10. The decoding of base64 encoded attachments/data ends when either the max mime depth or maximum MIME sessions (calculated using max mime depth and max mime mem) is reached or when the encoded data ends. This not normally printed out with the conﬁguration because it can print so much data. 15. Multiple base64 encoded MIME attachments/data in one packet are pipelined. The decoded data is available for detection using the rule option file data:mime. 12. print cmds List all commands understood by the preprocessor. max header line len <int> Alert if an SMTP DATA header line is longer than this value. 14.

Within FTP/Telnet. inspection type This indicates whether to operate in stateful or stateless mode. a particular session will be noted as encrypted and not inspected any further. FTP Client. which should allow the user to emulate any type of FTP server or FTP Client. and FTP Server.2.
Global Conﬁguration The global conﬁguration deals with conﬁguration options that determine the global functioning of FTP/Telnet. The default is to run FTP/Telnet in stateful inspection mode. FTP/Telnet will decode the stream. there are four areas of conﬁguration: Global. identifying FTP commands and responses and Telnet escape sequences and normalize the ﬁelds. The following example gives the generic global conﬁguration format: Format preprocessor ftp_telnet: \ global \ inspection_type stateful \ encrypted_traffic yes \ check_encrypted You can only have a single global conﬁguration. FTP/Telnet has the capability to handle stateless processing. whereas in stateful mode. you’ll get an error if you try otherwise. checks for encrypted trafﬁc will occur on every packet. similar to that of HTTP Inspect (See 2.6). check encrypted Instructs the preprocessor to continue to check an encrypted session for a subsequent command to cease encryption. Telnet. Users can conﬁgure individual FTP servers and clients with a variety of options. The FTP/Telnet global conﬁguration must appear before the other three areas of conﬁguration. encrypted traffic <yes|no> This option enables detection and alerting on encrypted Telnet and FTP command channels. The presence of the option indicates the option itself is on. FTP/Telnet has a very “rich” user conﬁguration. meaning it only looks for information on a packet-bypacket basis. This argument speciﬁes whether the user wants the conﬁguration option to generate a ftptelnet alert or not.8 FTP/Telnet Preprocessor
FTP/Telnet is an improvement to the Telnet decoder and provides stateful inspection capability for both FTP and Telnet data streams.2. 3. FTP/Telnet works on both client requests and server responses.
! △NOTE
Some conﬁguration options have an argument of yes or no. meaning it looks for information and handles reassembled data correctly. while the yes/no argument applies to the alerting functionality associated with that option. Conﬁguration 1.2.
! △NOTE
When inspection type is in stateless mode. 70
. 2.

ports {<port> [<port>< . Being that FTP uses the Telnet protocol on the control connection. 2. The following example gives the generic telnet conﬁguration format: Format preprocessor ftp_telnet_protocol: \ telnet \ ports { 23 } \ normalize \ ayt_attack_thresh 6 \ detect_anomalies
There should only be a single telnet conﬁguration.. and subsequent instances will override previously set values.. SSH tunnels cannot be decoded. 4. detect anomalies In order to support certain options. Typically port 23 will be included. It is only applicable when the mode is stateful.Example Global Conﬁguration preprocessor ftp_telnet: \ global inspection_type stateful encrypted_traffic no Telnet Conﬁguration The telnet conﬁguration deals with conﬁguration options that determine the functioning of the Telnet portion of the preprocessor. it is also susceptible to this behavior. Example Telnet Conﬁguration preprocessor ftp_telnet_protocol: \ telnet ports { 23 } normalize ayt_attack_thresh 6 FTP Server Conﬁguration There are two types of FTP server conﬁgurations: default and by IP address. normalize This option tells the preprocessor to normalize the telnet trafﬁc by eliminating the telnet escape sequences. the telnet decode preprocessor. Per the Telnet RFC. 3. certain implementations of Telnet servers will ignore the SB without a corresponding SE. This is anomalous behavior which could be an evasion case. Telnet supports subnegotiation. Rules written with ’raw’ content options will ignore the normalized buffer that is created when this option is in use. The detect anomalies option enables alerting on Telnet SB without the corresponding SE. Conﬁguration 1. >]} This is how the user conﬁgures which ports to decode as telnet trafﬁc. subnegotiation begins with SB (subnegotiation begin) and must end with an SE (subnegotiation end). so adding port 22 will only yield false positives. However.
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. It functions similarly to its predecessor. ayt attack thresh < number > This option causes the preprocessor to alert when the number of consecutive telnet Are You There (AYT) commands reaches the number speciﬁed.

Default This conﬁguration supplies the default server conﬁguration for any FTP server that is not individually conﬁgured. Most of your FTP servers will most likely end up using the default conﬁguration. Example Default FTP Server Conﬁguration preprocessor ftp_telnet_protocol: \ ftp server default ports { 21 } Refer to 74 for the list of options set in default ftp server conﬁguration. Conﬁguration by IP Address This format is very similar to “default”, the only difference being that speciﬁc IPs can be conﬁgured. Example IP speciﬁc FTP Server Conﬁguration preprocessor _telnet_protocol: \ ftp server 10.1.1.1 ports { 21 } ftp_cmds { XPWD XCWD } FTP Server Conﬁguration Options 1. ports {<port> [<port>< ... >]} This is how the user conﬁgures which ports to decode as FTP command channel trafﬁc. Typically port 21 will be included. 2. print cmds During initialization, this option causes the preprocessor to print the conﬁguration for each of the FTP commands for this server. 3. ftp cmds {cmd[cmd]} The preprocessor is conﬁgured to alert when it sees an FTP command that is not allowed by the server. This option speciﬁes a list of additional commands allowed by this server, outside of the default FTP command set as speciﬁed in RFC 959. This may be used to allow the use of the ’X’ commands identiﬁed in RFC 775, as well as any additional commands as needed. For example: ftp_cmds { XPWD XCWD XCUP XMKD XRMD } 4. def max param len <number> This speciﬁes the default maximum allowed parameter length for an FTP command. It can be used as a basic buffer overﬂow detection. 5. alt max param len <number> {cmd[cmd]} This speciﬁes the maximum allowed parameter length for the speciﬁed FTP command(s). It can be used as a more speciﬁc buffer overﬂow detection. For example the USER command – usernames may be no longer than 16 bytes, so the appropriate conﬁguration would be: alt_max_param_len 16 { USER } 6. chk str fmt {cmd[cmd]} This option causes a check for string format attacks in the speciﬁed commands.

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7. cmd validity cmd < fmt > This option speciﬁes the valid format for parameters of a given command. fmt must be enclosed in <>’s and may contain the following: Value int number char <chars> date <datefmt> Description Parameter must be an integer Parameter must be an integer between 1 and 255 Parameter must be a single character, one of <chars> Parameter follows format speciﬁed, where: n Number C Character [] optional format enclosed | OR {} choice of options . + - literal Parameter is a string (effectively unrestricted) Parameter must be a host/port speciﬁed, per RFC 959 Parameter must be a long host port speciﬁed, per RFC 1639 Parameter must be an extended host port speciﬁed, per RFC 2428 One of choices enclosed within, separated by | One of the choices enclosed within {}, optional value enclosed within []

A cmd validity line can be used to override these defaults and/or add a check for other commands. # This allows additional modes, including mode Z which allows for # zip-style compression. cmd_validity MODE < char ASBCZ > # Allow for a date in the MDTM command. cmd_validity MDTM < [ date nnnnnnnnnnnnnn[.n[n[n]]] ] string > MDTM is an off case that is worth discussing. While not part of an established standard, certain FTP servers accept MDTM commands that set the modiﬁcation time on a ﬁle. The most common among servers that do, accept a format using YYYYMMDDHHmmss[.uuu]. Some others accept a format using YYYYMMDDHHmmss[+—]TZ format. The example above is for the ﬁrst case (time format as speciﬁed in http://www.ietf.org/internetdrafts/draft-ietf-ftpext-mlst-16.txt) To check validity for a server that uses the TZ format, use the following: cmd_validity MDTM < [ date nnnnnnnnnnnnnn[{+|-}n[n]] ] string > 8. telnet cmds <yes|no> This option turns on detection and alerting when telnet escape sequences are seen on the FTP command channel. Injection of telnet escape sequences could be used as an evasion attempt on an FTP command channel. 73

Example Default FTP Client Conﬁguration preprocessor ftp_telnet_protocol: \ ftp client default bounce no max_resp_len 200 Conﬁguration by IP Address This format is very similar to “default”, the only difference being that speciﬁc IPs can be conﬁgured. Example IP speciﬁc FTP Client Conﬁguration preprocessor ftp_telnet_protocol: \ ftp client 10.1.1.1 bounce yes max_resp_len 500 FTP Client Conﬁguration Options 1. max resp len <number> This speciﬁes the maximum allowed response length to an FTP command accepted by the client. It can be used as a basic buffer overﬂow detection. 2. bounce <yes|no> This option turns on detection and alerting of FTP bounce attacks. An FTP bounce attack occurs when the FTP PORT command is issued and the speciﬁed host does not match the host of the client. 3. bounce to < CIDR,[port|portlow,porthi] > When the bounce option is turned on, this allows the PORT command to use the IP address (in CIDR format) and port (or inclusive port range) without generating an alert. It can be used to deal with proxied FTP connections where the FTP data channel is different from the client. A few examples: • Allow bounces to 192.162.1.1 port 20020 – ie, the use of PORT 192,168,1,1,78,52. bounce_to { 192.168.1.1,20020 } • Allow bounces to 192.162.1.1 ports 20020 through 20040 – ie, the use of PORT 192,168,1,1,78,xx, where xx is 52 through 72 inclusive. bounce_to { 192.168.1.1,20020,20040 } • Allow bounces to 192.162.1.1 port 20020 and 192.168.1.2 port 20030. bounce_to { 192.168.1.1,20020 192.168.1.2,20030 } • Allows bounces to IPv6 address fe8::5 port 59340.

IPv6 support must be enabled. bounce_to { fe8::5,59340 } 4. telnet cmds <yes|no> This option turns on detection and alerting when telnet escape sequences are seen on the FTP command channel. Injection of telnet escape sequences could be used as an evasion attempt on an FTP command channel. 5. ignore telnet erase cmds <yes|no> This option allows Snort to ignore telnet escape sequences for erase character (TNC EAC) and erase line (TNC EAL) when normalizing FTP command channel. Some FTP clients do not process those telnet escape sequences. 75

enable badmsgdir Enable alerts for trafﬁc ﬂowing the wrong direction. The SSH preprocessor should work by default.2. enable protomismatch Enables checking for the Protocol Mismatch exploit. Alerts at 19600 unacknowledged bytes within 20 encrypted packets for the Challenge-Response Overﬂow/CRC32 exploits. max client bytes < number > The number of unanswered bytes allowed to be transferred before alerting on Challenge-Response Overﬂow or CRC 32. After max encrypted packets is reached. Example Conﬁguration from snort. max server version len < number > The maximum number of bytes allowed in the SSH server version string before alerting on the Secure CRT server version string overﬂow. preprocessor ssh: \ server_ports { 22 } \ max_client_bytes 19600 \ max_encrypted_packets 20 \ enable_respoverflow \ enable_ssh1crc32
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. or if a client generates server trafﬁc. or else Snort will ignore the trafﬁc. 4. try increasing the number of required client bytes with max client bytes. For instance. 3. enable srvoverflow Enables checking for the Secure CRT exploit. This number must be hit before max encrypted packets packets are sent. If Challenge-Response Overﬂow or CRC 32 false positive. 9. autodetect Attempt to automatically detect SSH. enable recognition Enable alerts for non-SSH trafﬁc on SSH ports. 5. enable ssh1crc32 Enables checking for the CRC 32 exploit. This value can be set from 0 to 255. enable respoverflow Enables checking for the Challenge-Response Overﬂow exploit. The default is set to 25. This value can be set from 0 to 65535. the preprocessor will stop processing trafﬁc for a given session. if the presumed server generates client trafﬁc. 12. The default is set to 19600. 10. 8. Once max encrypted packets packets have been seen.conf Looks for attacks on SSH server port 22. 7. 6. max encrypted packets < number > The number of encrypted packets that Snort will inspect before ignoring a given SSH session. 11. The default is set to 80. This value can be set from 0 to 65535. Snort ignores the session to increase performance. The SSH vulnerabilities that Snort can detect all happen at the very beginning of an SSH session. enable paysize Enables alerts for invalid payload sizes.

SSLPP looks for a handshake followed by encrypted trafﬁc traveling to both sides.. Obsolete Record Types. By enabling the SSLPP to inspect port 443 and enabling the noinspect encrypted option. the user should use the ’trustservers’ option. enable rdata overflow Check for DNS Client RData TXT Overﬂow The DNS preprocessor does nothing if none of the 3 vulnerabilities it checks for are enabled. The available conﬁguration options are described below. 78
.conf Looks for trafﬁc on DNS server port 53. if a user knows that server-side encrypted data can be trusted to mark the session as encrypted. Check for the DNS Client RData overﬂow vulnerability. Do not alert on obsolete or experimental RData record types. no further inspection of the data on the connection is made. enable experimental types Alert on Experimental (per RFC 1035) Record Types 4. Once the trafﬁc is determined to be encrypted. the session is not marked as encrypted.. all alerts are disabled and the preprocessor checks trafﬁc on port 53. DNS looks at DNS Response trafﬁc over UDP and TCP and it requires Stream preprocessor to be enabled for TCP decoding. >]} This option speciﬁes the source ports that the DNS preprocessor should inspect trafﬁc. preprocessor dns: \ ports { 53 } \ enable_rdata_overflow
2. such as the handshake. ports {<port> [<port>< . By default. In some cases. especially when packets may be missed. SSL is used over port 443 as HTTPS. Conﬁguration By default. Examples/Default Conﬁguration from snort. and it will cease operation on a session if it loses state because of missing data (dropped packets). 2. Therefore. Verifying that faultless encrypted trafﬁc is sent from both endpoints ensures two things: the last client-side handshake packet was not crafted to evade Snort.10 DNS
The DNS preprocessor decodes DNS Responses and can detect the following exploits: DNS Client RData Overﬂow. documented below. Typically. It will not operate on TCP sessions picked up midstream. 1. If one side responds with an indication that something has failed. The SSL Dynamic Preprocessor (SSLPP) decodes SSL and TLS trafﬁc and optionally determines if and when Snort should stop inspection of it. and that the trafﬁc is legitimately encrypted. enable obsolete types Alert on Obsolete (per RFC 1035) Record Types 3. the only observed response from one endpoint will be TCP ACKs.2. and Experimental Record Types.11 SSL/TLS
Encrypted trafﬁc should be ignored by Snort for both performance reasons and to reduce false positives. only the SSL handshake of each connection will be inspected.2.2.

Examples/Default Conﬁguration from snort.2" Examples
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. Lists of identiﬁers are OR’ed together. multiple ssl version rule options should be used. The list of version identiﬁers are below. 3. via a comma separated list. preprocessor ssl: noinspect_encrypted Rule Options The following rule options are supported by enabling the ssl preprocessor: ssl_version ssl_state ssl version The ssl version rule option tracks the version negotiated between the endpoints of the SSL encryption. ports {<port> [<port>< . Default is off. To check for two or more SSL versions in use simultaneously. noinspect encrypted Disable inspection on trafﬁc that is encrypted..Conﬁguration 1. >]} This option speciﬁes which ports SSLPP will inspect trafﬁc on. version-list version = ["!"] "sslv2" | "sslv3" | "tls1.. Use this option for slightly better performance if you trust that your servers are not compromised.conf Enables the SSL preprocessor and tells it to disable inspection on encrypted trafﬁc. By default. and more than one identiﬁer can be speciﬁed. The option will match if any one of the OR’ed versions are used in the SSL connection. Default is off. Syntax
ssl_version: <version-list> version-list = version | version . SSLPP watches the following ports: • 443 HTTPS • 465 SMTPS • 563 NNTPS • 636 LDAPS • 989 FTPS • 992 TelnetS • 993 IMAPS • 994 IRCS • 995 POPS 2. This requires the noinspect encrypted option to be useful.1" | "tls1. trustservers Disables the requirement that application (encrypted) data must be observed on both sides of the session before a session is marked encrypted.0" | "tls1.

The preprocessor will use this list when detecting ARP cache overwrite attacks. When no arguments are speciﬁed to arpspoof. Specify one host IP MAC combo per line. ssl_state:!server_hello. multiple rules using the ssl state rule option should be used. and inconsistent Ethernet to IP mapping. When ”-unicast” is speciﬁed as the argument of arpspoof.ssl_version:sslv3. The option will match if the connection is currently in any one of the OR’ed states. the preprocessor checks for unicast ARP requests. and are OR’ed together.
ssl state The ssl state rule option tracks the state of the SSL encryption during the process of hello and key exchange. Format preprocessor arpspoof[: -unicast] preprocessor arpspoof_detect_host: ip mac Option ip mac Description IP address.
2.2. preprocessor arpspoof
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.0.server_keyx. The preprocessor merely looks for Ethernet address inconsistencies. When inconsistency occurs. Alert SID 4 is used in this case. An alert with GID 112 and SID 1 will be generated if a unicast ARP request is detected. Specify a pair of IP and hardware address as the argument to arpspoof detect host. ssl_version:tls1.
Example Conﬁguration The ﬁrst example conﬁguration does neither unicast detection nor ARP mapping monitoring.tls1. via a comma separated list.2. The list of states are below. state-list state = ["!"] "client_hello" | "server_hello" | "client_keyx" | "server_keyx" | "unknown" Examples ssl_state:client_hello. To ensure the connection has reached each of a set of states.tls1. ssl_version:!sslv2.12 ARP Spoof Preprocessor
The ARP spoof preprocessor decodes ARP packets and detects ARP attacks.1. the preprocessor inspects Ethernet addresses and the addresses in the ARP packets. The Ethernet address corresponding to the preceding IP. Syntax
ssl_state: <state-list> state-list = state | state . The host with the IP address should be on the same layer 2 segment as Snort is. ssl_state:client_keyx. More than one state can be speciﬁed. unicast ARP requests. an alert with GID 112 and SID 2 or 3 is generated.

reduce false positives and reduce the count and complexity of DCE/RPC based rules. Write and Close. preprocessor arpspoof: -unicast preprocessor arpspoof_detect_host: 192.168. Read Block Raw and Read AndX. i. Some important differences: Named pipe instance tracking A combination of valid login handle or UID. Samba 3.40. however. SMB desegmentation is performed for the following commands that can be used to transport DCE/RPC requests and responses: Write. stream5.0. Read.2. The binding between these is dependent on OS/software version. the frag3 preprocessor should be enabled and conﬁgured. the dcerpc2 preprocessor will enable stream reassembly for that session if necessary.1 proxy and server.168. if the TID used in creating the FID is deleted (via a tree disconnect). servers or autodetecting. either through conﬁgured ports.1 f0:0f:00:f0:0f:00 preprocessor arpspoof_detect_host: 192. UDP and RPC over HTTP v. TCP.2.e.40. Samba greater than 3. The following transports are supported for DCE/RPC: SMB.40.2 f0:0f:00:f0:0f:01
2.The next example conﬁguration does not do unicast detection but monitors ARP mapping for hosts 192.2 f0:0f:00:f0:0f:01 The third example conﬁguration has unicast detection enabled.168.22
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. along with a valid FID can be used to make a request. share handle or TID and ﬁle/named pipe handle or FID must be used to write data to a named pipe.168. no more requests can be written to that named pipe instance.168. Write Block Raw.22 and earlier Any valid UID and TID. New rule options have been implemented to improve performance.40.40. If it is decided that a session is SMB or DCE/RPC. Target Based There are enough important differences between Windows and Samba versions that a target based approach has been implemented. the FID that was created using this TID becomes invalid.1 and 192. • Stream reassembly must be performed for TCP sessions. i. • IP defragmentation should be enabled.e.0.168.13 DCE/RPC 2 Preprocessor
The main purpose of the preprocessor is to perform SMB desegmentation and DCE/RPC defragmentation to avoid rule evasion using these techniques.e. The preprocessor requires a session tracker to keep its data. preprocessor arpspoof preprocessor arpspoof_detect_host: 192. Dependency Requirements For proper functioning of the preprocessor: • Stream session tracking must be enabled. Transaction. Transaction Secondary.1 f0:0f:00:f0:0f:00 preprocessor arpspoof_detect_host: 192. i.40. Write AndX.

Windows 2003 Windows XP Windows Vista These Windows versions require strict binding between the UID. Multiple Transaction requests can be made simultaneously to the same named pipe.e. It also follows Samba greater than 3.22 and earlier. These requests can also be segmented with Transaction Secondary commands. having the same FID) are ﬁelds in the SMB header representing a process id (PID) and multiplex id (MID). It is necessary to track this so as not to munge these requests together (which would be a potential evasion opportunity). If the TID used to create the FID is deleted (via a tree disconnect). i. data is written to the named pipe as it is received by the server. on the other hand.0. TID and FID used to make a request to a named pipe instance. does not accept: Open Write And Close Read Read Block Raw Write Block Raw Windows (all versions) Accepts all of the above commands under an IPC$ tree.g. 82
. However. no binding. An evasion possibility would be accepting a fragment in a request that the server won’t accept that gets sandwiched between an exploit. no more requests can be written to that named pipe instance. However.e. e. the client has to explicitly send one of the Read* requests to tell the server to send the response and (2) a Transaction request is not written to the named pipe until all of the data is received (via potential Transaction Secondary requests) whereas with the Write* commands. Transaction tracking The differences between a Transaction request and using one of the Write* commands to write data to a named pipe are that (1) a Transaction performs the operations of a write and a read from the named pipe.22 in that deleting the UID or TID used to create the named pipe instance also invalidates it. whereas in using the Write* commands. deleting either the UID or TID invalidates the FID. requests after that follow the same binding as Samba 3. Both the UID and TID used to open the named pipe instance must be used when writing data to the same named pipe instance. If the UID used to create the named pipe instance is deleted (via a Logoff AndX). Therefore. is very lax and allows some nonsensical combinations. Accepted SMB commands Samba in particular does not recognize certain commands under an IPC$ tree.Any valid TID. only the UID used in opening the named pipe can be used to make a request using the FID handle to the named pipe instance. multiple logins and tree connects (only one place to return handles for these). along with a valid FID can be used to make a request. Samba. An MID represents different sub-processes within a process (or under a PID). since it is necessary in making a request to the named pipe. Ultimately. The PID represents the process this request is a part of. Samba (all versions) Under an IPC$ tree. Segments for each ”thread” are stored separately and written to the named pipe when all segments are received. the FID becomes invalid. the FID that was created using this TID becomes invalid. AndX command chaining Windows is very strict in what command combinations it allows to be chained. login/logoff and tree connect/tree disconnect. What distinguishes them (when the same named pipe is being written to. i.0. we don’t want to keep track of data that the server won’t accept. i. Windows 2000 Windows 2000 is interesting in that the ﬁrst request to a named pipe must use the same binding as that of the other Windows versions.e.

Samba (all versions) Windows 2000 Windows 2003 Windows XP The opnum that is ultimately used for the request is contained in the last fragment. Samba later than 3.Windows (all versions) Uses a combination of PID and MID to deﬁne a ”thread”. Any binding after that must use the Alter Context request. Samba (all versions) The byte order of the stub data is that which is used in the request carrying the stub data. DCE/RPC Fragmented requests . only one Bind can ever be made on a session whether or not it succeeds or fails. DCE/RPC Fragmented requests . The opnum ﬁeld in any other fragment can contain any value. Samba (all versions) The context id that is ultimately used for the request is contained in the last fragment.20 and earlier Any amount of Bind requests can be made. DCE/RPC Stub data byte order The byte order of the stub data is determined differently for Windows and Samba. 83
.Context ID Each fragment in a fragmented request carries the context id of the bound interface it wants to make the request to. The context id ﬁeld in any other fragment can contain any value.0. The context id ﬁeld in any other fragment can contain any value. Samba 3. If a Bind after a successful Bind is made.0. The opnum ﬁeld in any other fragment can contain any value. all previous interface bindings are invalidated. Windows (all versions) For all of the Windows versions.Operation number Each fragment in a fragmented request carries an operation number (opnum) which is more or less a handle to a function offered by the interface. Windows Vista The opnum that is ultimately used for the request is contained in the ﬁrst fragment. Samba (all versions) Uses just the MID to deﬁne a ”thread”. Windows (all versions) The context id that is ultimately used for the request is contained in the ﬁrst fragment. all previous interface bindings are invalidated.20 Another Bind request can be made if the ﬁrst failed and no interfaces were successfully bound to. Windows (all versions) The byte order of the stub data is that which was used in the Bind request. If another Bind is made. Multiple Bind Requests A Bind request is the ﬁrst request that must be made in a connection-oriented DCE/RPC session in order to specify the interface/interfaces that one wants to communicate with.

(See Events section for an enumeration and explanation of events. Default is to do defragmentation. By default this value is turned off.Conﬁguration The dcerpc2 preprocessor has a global conﬁguration and one or more server conﬁgurations. disable defrag Tells the preprocessor not to do DCE/RPC defragmentation.) memcap Only one event. smb 84
. When the preprocessor is disabled only the memcap option is applied when speciﬁed with the conﬁguration. events Speciﬁes the classes of events to enable. Option syntax Option memcap disable defrag max frag len events reassemble threshold disabled
memcap max-frag-len events pseudo-event event-list event re-thresh = = = = = = =
Argument <memcap> NONE <max-frag-len> <events> <re-thresh> NONE
Required NO NO NO NO NO NO
Default memcap 102400 OFF OFF OFF OFF OFF
1024-4194303 (kilobytes) 1514-65535 pseudo-event | event | ’[’ event-list ’]’ "none" | "all" event | event ’. max frag len Speciﬁes the maximum fragment size that will be added to the defragmention module. Default is 100 MB. The allowed range for this option is 1514 . If a fragment is greater than this size. Global Conﬁguration preprocessor dcerpc2 The global dcerpc2 conﬁguration is required. Only one global dcerpc2 conﬁguration can be speciﬁed. disabled Disables the preprocessor. Default is set to -1.’ event-list "memcap" | "smb" | "co" | "cl" 0-65535
Option explanations memcap Speciﬁes the maximum amount of run-time memory that can be allocated. Run-time memory includes any memory allocated after conﬁguration.65535. If the memcap is reached or exceeded. alert. The global preprocessor conﬁguration name is dcerpc2 and the server preprocessor conﬁguration name is dcerpc2 server. it is truncated before being added to the defragmentation module.

max_frag_len 14440 disable_defrag. If a net conﬁguration matches. The default and net options are mutually exclusive. For any dcerpc2 server conﬁguration. Alert on events related to connectionless DCE/RPC processing. Zero or more net conﬁgurations can be speciﬁed. Note that port and ip variables deﬁned in snort. disable this option.conf CANNOT be used. The net option supports IPv6 addresses. events [memcap. If no default conﬁguration is speciﬁed.Alert on events related to SMB processing. At most one default conﬁguration can be speciﬁed. A dcerpc2 server conﬁguration must start with default or net options. This option is useful in inline mode so as to potentially catch an exploit early before full defragmentation is done. co] events [memcap. cl] reassemble_threshold 500
Conﬁguration examples
preprocessor preprocessor preprocessor preprocessor preprocessor preprocessor dcerpc2 dcerpc2: dcerpc2: dcerpc2: dcerpc2: dcerpc2:
memcap 500000 max_frag_len 16840. events [memcap. default values will be used for the default conﬁguration. it will override the default conﬁguration. reassemble threshold Speciﬁes a minimum number of bytes in the DCE/RPC desegmentation and defragmentation buffers before creating a reassembly packet to send to the detection engine. co. Option examples
memcap 30000 max_frag_len 16840 events none events all events smb events co events [co] events [smb. When processing DCE/RPC trafﬁc. A value of 0 supplied as an argument to this option will. if non-required options are not speciﬁed. Option syntax
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. smb. Default is disabled. events smb memcap 50000. cl]. the defaults will be used. smb. memcap 300000. A net conﬁguration matches if the packet’s server IP address matches an IP address or net speciﬁed in the net conﬁguration. co. the default conﬁguration is used if no net conﬁgurations match. Alert on events related to connection-oriented DCE/RPC processing. cl Stands for connectionless DCE/RPC. co Stands for connection-oriented DCE/RPC. in effect. smb] reassemble_threshold 500
Default global conﬁguration
preprocessor dcerpc2: memcap 102400
Server Conﬁguration preprocessor dcerpc2_server The dcerpc2 server conﬁguration is optional.

In this case the preprocessor is concerned with the server response. The preprocessor will alert if the total data count speciﬁed in the SMB command header is less than the data size speciﬁed in the SMB command header. have a ﬁeld containing the total amount of data to be transmitted. The preprocessor will alert if the remaining NetBIOS packet length is less than the size of the SMB command data size speciﬁed in the command header. Some commands require a speciﬁc format for the data. they are responded to in the order they were sent. SMB commands have pretty speciﬁc word counts and if the preprocessor sees a command with a word count that doesn’t jive with that command. The server response. however. Unlike the Tree Connect AndX response. the preprocessor will alert. the preprocessor will alert. the preprocessor has to queue the requests up and wait for a server response to determine whether or not an IPC share was successfully connected to (which is what the preprocessor is interested in). If this offset puts us before data that has already been processed or after the end of payload. Note that since the preprocessor does not yet support SMB2. The preprocessor will alert if the byte count minus a predetermined amount based on the SMB command is not equal to the data size. There should be under normal circumstances no more than a few pending tree connects at a time and the preprocessor will alert if this number is excessive. so it need to be queued with the request and dequeued with the response. the preprocessor will alert. After a client is done writing data using the Write* commands. If a command requires this and the byte count is less than the minimum required byte count for that command. Some commands. especially the commands from the SMB Core implementation require a data format ﬁeld that speciﬁes the kind of data that will be coming next.) The preprocessor will alert if the total amount of data sent in a transaction is greater than the total data count speciﬁed in the SMB command header. (Total data count must always be greater than or equal to current data size.4 5
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The SMB id does not equal \xffSMB.) Some of the Core Protocol commands (from the initial SMB implementation) require that the byte count be some value greater than the data size exactly.
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. The preprocessor will alert if the NetBIOS Session Service length ﬁeld contains a value less than the size of an SMB header. the preprocessor will alert. The Read* request contains the ﬁle id associated with a named pipe instance that the preprocessor will ultimately send the data to. id of \xfeSMB is turned away before an eventable point is reached. The preprocessor will alert if the remaining NetBIOS packet length is less than the size of the SMB command header to be decoded. There should be under normal circumstances no more than a few pending Read* requests at a time and the preprocessor will alert if this number is excessive. For the Tree Connect command (and not the Tree Connect AndX command). it issues a Read* command to the server to tell it to send a response to the data it has written. (The byte count must always be greater than or equal to the data size. does not contain this ﬁle id. The preprocessor will alert if the format is not that which is expected for that command. there is no indication in the Tree Connect response as to whether the share is IPC or not. If this ﬁeld is zero. The word count of the command header is invalid. The preprocessor will alert if the remaining NetBIOS packet length is less than the size of the SMB command byte count speciﬁed in the command header. Some commands require a minimum number of bytes after the command header. such as Transaction. Some SMB commands. The preprocessor will alert if the byte count speciﬁed in the SMB command header is less than the data size speciﬁed in the SMB command. The preprocessor will alert if the number of chained commands in a single request is greater than or equal to the conﬁgured amount (default is 3). If multiple Read* requests are sent to the server. Many SMB commands have a ﬁeld containing an offset from the beginning of the SMB header to where the data the command is carrying starts.

A Logoff AndX request is sent by the client to indicate it wants to end the session and invalidate the login handle. It is anomalous behavior to attempt to change the byte order mid-session. essentially connects to a share and disconnects from the same share in the same request and is anomalous behavior. A Tree Connect AndX command is used to connect to a share. Windows does not allow this behavior. It looks for a Tree Connect or Tree Connect AndX to the share. Windows does not allow this behavior.) The Close command is used to close that ﬁle or named pipe. essentially logins in and logs off in the same request and is anomalous behavior. The combination of a Open AndX or Nt Create AndX command with a chained Close command. The preprocessor will alert if it sees this. The combination of a Session Setup AndX command with a chained Logoff AndX command. The preprocessor will alert if it sees this. The byte order of the request data is determined by the Bind in connection-oriented DCE/RPC for Windows. With AndX command chaining it is possible to chain multiple Tree Connect AndX commands within the same request. there are no context items speciﬁed. the server responds (if the client successfully authenticates) which a user id or login handle. 90
35 36
. The combination of a Tree Connect AndX command with a chained Tree Disconnect command. The preprocessor will alert if in a Bind or Alter Context request. only one place in the SMB header to return a login handle (or Uid). there are no transfer syntaxes to go with the requested interface. The preprocessor will alert if in a Bind or Alter Context request. There is. The Tree Disconnect command is used to disconnect from that share. There is. The preprocessor will alert if a fragment is larger than the maximum negotiated fragment length. The preprocessor will alert if the remaining fragment length is less than the remaining packet size. The preprocessor will alert if the connection-oriented DCE/RPC PDU type contained in the header is not a valid PDU type. With commands that are chained after a Session Setup AndX request. the login handle returned by the server is used for the subsequent chained commands. The preprocessor will alert if the connection-oriented DCE/RPC minor version contained in the header is not equal to 0. (The preprocessor is only interested in named pipes as this is where DCE/RPC requests are written to. This is used by the client in subsequent requests to indicate that it has authenticated. This is anomalous behavior and the preprocessor will alert if it happens. The preprocessor will alert if it sees any of the invalid SMB shares conﬁgured. however Samba does. Most evasion techniques try to fragment the data as much as possible and usually each fragment comes well below the negotiated transmit size. The preprocessor will alert if the fragment length deﬁned in the header is less than the size of the header. however. The preprocessor will alert if a non-last fragment is less than the size of the negotiated maximum fragment length. however. When a Session Setup AndX request is sent to the server. however Samba does. This is anomalous behavior and the preprocessor will alert if it happens. essentially opens and closes the named pipe in the same request and is anomalous behavior. only one place in the SMB header to return a tree handle (or Tid). The preprocessor will alert if it sees this.21
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With AndX command chaining it is possible to chain multiple Session Setup AndX commands within the same request. An Open AndX or Nt Create AndX command is used to open/create a ﬁle or named pipe.
Connection-oriented DCE/RPC events SID 27 28 29 30 31 32 33 34 Description The preprocessor will alert if the connection-oriented DCE/RPC major version contained in the header is not equal to 5.

wrapping the sequence number space produces strange behavior from the server. The preprocessor will alert if the opnum changes in a fragment mid-request. specify one or more service interfaces to bind to. In testing.
Rule Options New rule options are supported by enabling the dcerpc2 preprocessor: dce_iface dce_opnum dce_stub_data New modiﬁers to existing byte test and byte jump rule options: byte_test:dce byte_jump:dce dce iface For DCE/RPC based rules it has been necessary to set ﬂow-bits based on a client bind to a service to avoid false positives. When a client sends a bind request to the server. If a request is fragmented. The server will respond with the interface UUIDs it accepts as valid and will allow the client to make requests to those services. so this should be considered anomalous behavior.
Connectionless DCE/RPC events SID 40 41 42 43 Description The preprocessor will alert if the connectionless DCE/RPC major version is not equal to 4. a rule can simply ask the preprocessor. however. Instead of using ﬂow-bits. The preprocessor will alert if the context id changes in a fragment mid-request. The operation number speciﬁes which function the request is calling on the bound interface.one for big endian and 91
. it will specify the context id so the server knows what service the client is making a request to. The preprocessor will alert if the connectionless DCE/RPC PDU type is not a valid PDU type. This can eliminate false positives where more than one service is bound to successfully since the preprocessor can correlate the bind UUID to the context id used in the request. Each interface is represented by a UUID.37 38
39
The call id for a set of fragments in a fragmented request should stay the same (it is incremented for each complete request). this number should stay the same for all fragments. A DCE/RPC request can specify whether numbers are represented as big endian or little endian. this number should stay the same for all fragments. It is necessary for a client to bind to a service before being able to make a call to it. it can. When a client makes a request. The preprocessor will alert if the packet data length is less than the size of the connectionless header. If a request if fragmented. The context id is a handle to a interface that was bound to. The preprocessor will alert if the sequence number uses in a request is the same or less than a previously used sequence number on the session. The preprocessor will alert if it changes in a fragment mid-request. using this rule option. whether or not the client has bound to a speciﬁc interface UUID and whether or not this client request is making a request to it. Each interface UUID is paired with a unique index (or context id) that future requests can use to reference the service that the client is making a call to. The representation of the interface UUID is different depending on the endianness speciﬁed in the DCE/RPC previously requiring two rules .

any_frag. This tracking is required so that when a request is processed. The preprocessor eliminates the need for two rules by normalizing the UUID. The any frag argument says to evaluate for middle and last fragments as well. Some versions of an interface may not be vulnerable to a certain exploit. any_frag. equal to (’=’) or not equal to (’!’) the version speciﬁed. =1. since subsequent fragments will contain data deeper into the DCE/RPC request. any_frag]. Also. if the any frag option is used to specify evaluating on all fragments. This can be a source of false positives in fragmented DCE/RPC trafﬁc. greater than (’>’). However. uuid hexlong hexshort hexbyte operator version = = = = = = hexlong ’-’ hexshort ’-’ hexshort ’-’ 2hexbyte ’-’ 6hexbyte 4hexbyte 2hexbyte 2HEXDIGIT ’<’ | ’>’ | ’=’ | ’!’ 0-65535
Examples
dce_iface:4b324fc8-1670-01d3-1278-5a47bf6ee188. Syntax
dce_iface:<uuid>[. hexlong and hexshort will be speciﬁed and interpreted to be in big endian order (this is usually the default way an interface UUID will be seen and represented). For each Bind and Alter Context request. This option requires tracking client Bind and Alter Context requests as well as server Bind Ack and Alter Context responses for connection-oriented DCE/RPC in the preprocessor. As an example. Many checks for data in the DCE/RPC request are only relevant if the DCE/RPC request is a ﬁrst fragment (or full request). the following Messenger interface UUID as taken off the wire from a little endian Bind request: |f8 91 7b 5a 00 ff d0 11 a9 b2 00 c0 4f b6 e6 fc| must be written as: 5a7b91f8-ff00-11d0-a9b2-00c04fb6e6fc The same UUID taken off the wire from a big endian Bind request: |5a 7b 91 f8 ff 00 11 d0 a9 b2 00 c0 4f b6 e6 fc| must be written the same way: 5a7b91f8-ff00-11d0-a9b2-00c04fb6e6fc
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. since the beginning of subsequent fragments are already offset some length from the beginning of the request.e. An interface contains a version. dce_iface:4b324fc8-1670-01d3-1278-5a47bf6ee188. Flags (and a ﬁeld in the connectionless header) are set in the DCE/RPC header to indicate whether the fragment is the ﬁrst. <2. the client speciﬁes a list of interface UUIDs along with a handle (or context id) for each interface UUID that will be used during the DCE/RPC session to reference the interface. Also. dce_iface:4b324fc8-1670-01d3-1278-5a47bf6ee188.
This option is used to specify an interface UUID. The server response indicates which interfaces it will allow the client to make requests to . <operator><version>][. A rule which is looking for data. not a fragment) since most rules are written to start at the beginning of a request.it either accepts or rejects the client’s wish to bind to a certain interface. a DCE/RPC request can be broken up into 1 or more fragments. will be looking at the wrong data on a fragment other than the ﬁrst. the context id used in the request can be correlated with the interface UUID it is a handle for. By default it is reasonable to only evaluate if the request is a ﬁrst fragment (or full request). a middle or the last fragment. i.one for little endian. say 5 bytes into the request (maybe it’s a length ﬁeld). by default the rule will only be evaluated for a ﬁrst fragment (or full request. Optional arguments are an interface version and operator to specify that the version be less than (’<’). dce_iface:4b324fc8-1670-01d3-1278-5a47bf6ee188.

dce_opnum:15. the best (meaning longest) pattern will be used. opnum-list opnum-item opnum-range opnum = = = = opnum-item | opnum-item ’. This option matches if there is DCE/RPC stub data. 93
.This option matches if the speciﬁed interface UUID matches the interface UUID (as referred to by the context id) of the DCE/RPC request and if supplied. (1) if the rule option flow:to server|from client is used. This option matches if any one of the opnums speciﬁed match the opnum of the DCE/RPC request. 18-20.
This option is used to specify an opnum (or operation number). |05 00| will be inserted into the fast pattern matcher. opnum range or list containing either or both opnum and/or opnum-range. For UDP rules. Syntax
dce_opnum:<opnum-list>. dce stub data Since most netbios rules were doing protocol decoding only to get to the DCE/RPC stub data. Note that if the rule already has content rule options in it. it will unequivocally be used over the above mentioned patterns.’ opnum-list opnum | opnum-range opnum ’-’ opnum 0-65535
Examples
dce_opnum:15. |05 00 00| will be inserted into the fast pattern matcher. |05 00 02| will be inserted into the fast pattern matcher and (3) if the ﬂow isn’t known. regardless of preceding rule options. The opnum of a DCE/RPC request will be matched against the opnums speciﬁed with this option. the interface UUID. dce_opnum:15.
! △NOTE
Using this rule option will automatically insert fast pattern contents into the fast pattern matcher. If a content in the rule uses the fast pattern rule option. This option will not match if the fragment is not a ﬁrst fragment (or full request) unless the any frag option is supplied in which case only the interface UUID and version need match. This reduces the number of rule option checks and the complexity of the rule. dce opnum The opnum represents a speciﬁc function call to an interface. This option takes no arguments. (2) if the rule option flow:from server|to client is used. For TCP rules. It is likely that an exploit lies in the particular DCE/RPC function call. this option will alleviate this need and place the cursor at the beginning of the DCE/RPC stub data. There are no arguments to this option. i. dce_opnum:15-18.dce iface) usually we want to know what function call it is making to that service. in both big and little endian format will be inserted into the fast pattern matcher. 20-22. After is has been determined that a client has bound to a speciﬁc interface and is making a request to it (see above .e. the remote procedure call or function call data.
This option is used to place the cursor (used to walk the packet payload in rules processing) at the beginning of the DCE/RPC stub data. the version operation is true. Note that a defragmented DCE/RPC request will be considered a full request. Example
dce_stub_data. 17.

Snort recognizes Social Security numbers issued up through November 2009. ssn file A Social Security number is broken up into 3 sections: Area (3 digits). where an organization’s regulations may prevent them from seeing unencrypted numbers. the Social Security Administration publishes a list of which Group numbers are in use for each Area. and the rule options. A new rule option is provided by the preprocessor: sd_pattern
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. On a monthly basis. These numbers can be updated in Snort by supplying a CSV ﬁle with the new maximum Group numbers to use. This is only done on credit card & Social Security numbers.Dependencies The Stream5 preprocessor must be enabled for the Sensitive Data preprocessor to work. The preprocessor conﬁg starts with: preprocessor sensitive_data: Option syntax Option alert threshold mask output ssn file
alert_threshold =
Argument <number> NONE <filename>
1 . Group (2 digits). This option speciﬁes how many need to be detected before alerting. Preprocessor Conﬁguration Sensitive Data conﬁguration is split into two parts: the preprocessor conﬁg. and Serial (4 digits). By default.65535
Required NO NO NO
Default alert threshold 25 OFF OFF
Option explanations alert threshold The preprocessor will alert when any combination of PII are detected in a session.csv Rule Options Snort rules are used to specify which PII the preprocessor should look for. mask output This option replaces all but the last 4 digits of a detected PII with ”X”s. This should be set higher than the highest individual count in your ”sd pattern” rules. Example preprocessor conﬁg preprocessor sensitive_data: alert_threshold 25 \ mask_output \ ssn_file ssn_groups_Jan10.

. Discover. Other characters in the pattern will be matched literally. These numbers may have spaces.
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.and 16-digit credit card numbers. Social Security numbers without dashes separating the Area. \d \D \l \L \w \W {num}
! △NOTE\w in this rule option does NOT match underscores. Unlike PCRE. us social nodashes This pattern matches U. example: ”{3}” matches 3 digits. <pattern>. and American Express. or nothing in between groups. Custom PII types are deﬁned using a limited regex-style syntax. This covers Visa.This rule option speciﬁes what type of PII a rule should detect. then it is the deﬁnition of a custom PII pattern. email This pattern matches against email addresses. but the preprocessor will check matches against the list of currently allocated group numbers. The count is tracked across all packets in a session. and Serial sections.255 pattern = any string
Option Explanations count This dictates how many times a PII pattern must be matched for an alert to be generated. Mastercard.S. Group. The following special characters and escape sequences are supported: matches any digit matches any non-digit matches any letter matches any non-letter matches any alphanumeric character matches any non-alphanumeric character used to repeat a character or escape sequence ”num” times. If the pattern speciﬁed is not one of the above built-in patterns. \} matches { and } \? matches a question mark.
count = 1 . Social Security numbers. pattern This is where the pattern of the PII gets speciﬁed.S. SSNs have no check digits. This behaves in a greedy manner. \\ matches a backslash \{. and Serial sections. dashes. There are a few built-in patterns to choose from: credit card The ”credit card” pattern matches 15. The SSNs are expected to have dashes between the Area. Credit card numbers matched this way have their check digits veriﬁed using the Luhn algorithm. Group. example: ” ?” matches an optional space. ? makes the previous character or escape sequence optional. us social This pattern matches against 9-digit U. Syntax sd_pattern:<count>.

use the following when conﬁguring Snort: . 97
. • tos type of service (differentiated services) ﬁeld: clear this byte. gid:138. sd_pattern: 5.us_social. metadata:service smtp. • trim truncate packets with excess payload to the datagram length speciﬁed in the IP header + the layer 2 header (eg ethernet). Alerts when 2 social security numbers (with dashes) appear in a session. sid:1000. phone numbers. Trying to use other rule options with sd pattern will result in an error message. [trim] Base normalizations enabled with ”preprocessor normalize ip4” include: • TTL normalization if enabled (explained below). normalizations will only be enabled if the selected DAQ supports packet replacement and is operating in inline mode.
2.credit_card. [rf]. If a policy is conﬁgured for inline test or passive mode. any normalization statements in the policy conﬁg are ignored.Examples sd_pattern: 2./configure --enable-normalizer The normalize preprocessor is activated via the conf as outlined below. [tos].S.15 Normalizer
When operating Snort in inline mode. \ sd_pattern:4. Note that in the following. • rf reserved ﬂag: clear this bit on incoming packets.(\d{3})\d{3}-\d{4}. it is helpful to normalize packets to help minimize the chances of evasion. Optional normalizations include: • df don’t fragment: clear this bit on incoming packets.) Caveats sd pattern is not compatible with other rule options. • NOP all options octets. Alerts on 5 U. Also. rev:1. but don’t truncate below minimum frame length. There are also many new preprocessor and decoder rules to alert on or drop packets with ”abnormal” encodings. ﬁelds are cleared only if they are non-zero. To enable the normalizer. IP4 Normalizations IP4 normalizations are enabled with: preprocessor normalize_ip4: [df]. Rules using sd pattern must use GID 138.2. This is automatically disabled if the DAQ can’t inject packets. following the format (123)456-7890 Whole rule example: alert tcp $HOME_NET $HIGH_PORTS -> $EXTERNAL_NET $SMTP_PORTS \ (msg:"Credit Card numbers sent over email".

• opts NOP all option bytes other than maximum segment size. and any explicitly allowed with the allow keyword.. • Remove any data from RST packet. • opts if timestamp is present but invalid. You can allow options to pass by name or number. block the packet. Should also enable require 3whs. • opts clear TS ECR if ACK ﬂag is not set. • Clear the urgent ﬂag if the urgent pointer is not set. timestamp.. • Trim data to MSS. • urp urgent pointer: don’t adjust the urgent pointer if it is greater than payload length. • ecn packet clear ECN ﬂags on a per packet basis (regardless of negotiation). • opts MSS and window scale options are NOP’d if SYN ﬂag is not set. Any segments that can’t be properly reassembled will be dropped.255) <new_ttl> ::= (<min_ttl>+1. TTL Normalization TTL normalization pertains to both IP4 TTL (time-to-live) and IP6 (hop limit) and is only performed if both the relevant base normalization is enabled (as described above) and the minimum and new TTL values are conﬁgured. • opts trim payload length to MSS if longer. • Trim data to window.255) 99
. • Clear any option padding bytes. Optional normalizations include: • ips ensure consistency in retransmitted data (also forces reassembly policy to ”ﬁrst”). NOP the timestamp octets. • ecn stream clear ECN ﬂags if usage wasn’t negotiated.• Set the urgent pointer to the payload length if it is greater than the payload length. or valid but not negotiated. as follows: config min_ttl: <min_ttl> config new_ttl: <new_ttl> <min_ttl> ::= (1. window scaling. • opts if timestamp was negotiated but not present.

To change the rule type or action of a decoder/preprocessor rule.map under etc directory is also updated with new decoder and preprocessor rules. and have the names decoder.conf or the decoder or preprocessor rule type is drop.If new ttl ¿ min ttl. The gen-msg.conf. just comment it with a # or remove the rule completely from the ﬁle (commenting is recommended).rules and preprocessor. These ﬁles are updated as new decoder and preprocessor events are added to Snort. For example.rules To disable any rule.3 Decoder and Preprocessor Rules
Decoder and preprocessor rules allow one to enable and disable decoder and preprocessor events on a rule by rule basis. Note that this conﬁguration item was deprecated in 2.conf.decode for conﬁg options that control decoder events.
2. When TTL normalization is turned on the new ttl is set to 5 by default. Of course.
2. See doc/README...g. A packet will be dropped if either a decoder conﬁg drop option is in snort. decoder events will not be generated regardless of whether or not there are corresponding rules for the event. if config disable decode alerts is in snort. They also allow one to specify the rule type or action of a decoder or preprocessor event on a rule by rule basis.6: preprocessor stream5_tcp: min_ttl <#> By default min ttl = 1 (TTL normalization is disabled). deﬁne the path to where the rules are located and uncomment the include lines in snort. var PREPROC_RULE_PATH /path/to/preproc_rules .1 Conﬁguring
The following options to conﬁgure will enable decoder and preprocessor rules: $ .conf that reference the rules ﬁles. then if a packet is received with a TTL ¡ min ttl. just replace alert with the desired rule type. include $PREPROC_RULE_PATH/preprocessor. Decoder conﬁg options will still determine whether or not to generate decoder events.3. e.rules include $PREPROC_RULE_PATH/decoder. the drop cases only apply if Snort is running inline./configure --enable-decoder-preprocessor-rules The decoder and preprocessor rules are located in the preproc rules/ directory in the top level source tree. Any one of the following rule types can be used: alert log pass drop sdrop reject For example one can change: 100
. config enable decode drops. these options will take precedence over the event type of the rule. the TTL will be set to new ttl. Also note that if the decoder is conﬁgured to enable drops.rules respectively. To enable these rules in snort.8.

\ metadata: rule-type decode . rev: 1. See README.decode. classtype:protocol-command-decode.rules and preprocessor. the following conﬁg option in snort.) to drop (as well as alert on) packets where the Ethernet protocol is IPv4 but version ﬁeld in IPv4 header has a value other than 4. rev: 1. \ metadata: rule-type decode .) to drop ( msg: "DECODE_NOT_IPV4_DGRAM".conf.2 Reverting to original behavior
If you have conﬁgured snort to use decoder and preprocessor rules. you also have to remove the decoder and preprocessor rules and any reference to them from snort. gid: 116. The generator ids ( gid ) for different preprocessors and the decoder are as follows: Generator Id 105 106 112 116 119 120 122 123 124 125 126 127 128 129 131 132 133 134 137 139 Module Back Oriﬁce preprocessor RPC Decode preprocessor Arpspoof preprocessor Snort Decoder HTTP Inspect preprocessor ( Client ) HTTP Inspect preprocessor ( Server ) Portscan preprocessor Frag3 preprocessor SMTP preprocessor FTP (FTP) preprocessor FTP (Telnet) preprocessor ISAKMP preprocessor SSH preprocessor Stream5 preprocessor DNS preprocessor Skype preprocessor DceRpc2 preprocessor PPM preprocessor SSL preprocessor SDF preprocessor
2. README.3.conf will make Snort revert to the old behavior: config autogenerate_preprocessor_decoder_rules Note that if you want to revert to the old behavior.4 Event Processing
Snort provides a variety of mechanisms to tune event processing to suit your needs: 101
.gre and the various preprocessor READMEs for descriptions of the rules in decoder. gid: 116.
2. sid: 1. This option applies to rules not speciﬁed and the default behavior is to alert. otherwise they will be loaded.rules. classtype:protocol-command-decode.alert ( msg: "DECODE_NOT_IPV4_DGRAM". sid: 1.

allow a maximum of 100 successful simultaneous connections from any one IP address. This is covered in section 3.allow a maximum of 100 connection attempts per second from any one IP address.10. • Rate Filters You can use rate ﬁlters to change a rule action when the number or rate of events indicates a possible attack. sig_id 2. • Event Suppression You can completely suppress the logging of unintersting events. \ track by_src. timeout 10 102
. timeout 10 Example 2 . seconds 1. \ track by_src.4. apply_to <ip-list>] The options are described in the table below .all are required except apply to. sig_id <sid>. seconds 0. seconds <s>. This can be tuned to signiﬁcantly reduce false alarms. Multiple rate ﬁlters can be deﬁned on the same rule.1 Rate Filtering
rate filter provides rate based attack prevention by allowing users to conﬁgure a new action to take for a speciﬁed time when a given rate is exceeded. in which case they are evaluated in the order they appear in the conﬁguration ﬁle. \ track <by_src|by_dst|by_rule>. Format Rate ﬁlters are used as standalone conﬁgurations (outside of a rule) and have the following format: rate_filter \ gen_id <gid>. Examples Example 1 . sig_id 1.
2.7. and block further connection attempts from that IP address for 10 seconds: rate_filter \ gen_id 135. • Event Filters You can use event ﬁlters to reduce the number of logged events for noisy rules. and block further connections from that IP address for 10 seconds: rate_filter \ gen_id 135. \ new_action drop. which is optional. \ new_action drop.• Detection Filters You can use detection ﬁlters to specify a threshold that must be exceeded before a rule generates an event. \ timeout <seconds> \ [. \ count 100. \ count <c>. and the ﬁrst applicable action is taken. \ count 100. \ new_action alert|drop|pass|log|sdrop|reject.

the time period over which count is accrued. the maximum number of rule matches in s seconds before the rate ﬁlter limit to is exceeded. \ count <c>. \ track <by_src|by_dst>. This can be tuned to signiﬁcantly reduce false alarms.Option track by src | by dst | by rule
count c seconds s
new action alert | drop | pass | log | sdrop | reject timeout t
apply to <ip-list>
Description rate is tracked either by source IP address. \ track <by_src|by_dst>. even if the rate falls below the conﬁgured limit. There are 3 types of event ﬁlters: • limit Alerts on the 1st m events during the time interval. c must be nonzero value.4. then ignores any additional events during the time interval. restrict the conﬁguration to only to source or destination IP address (indicated by track parameter) determined by <ip-list>. For example. 0 seconds only applies to internal rules (gen id 135) and other use will produce a fatal error by Snort. sdrop and reject are conditionally compiled with GIDS. drop. • both Alerts once per time interval after seeing m occurrences of the event. and sdrop can be used only when snort is used in inline mode. track by rule and apply to may not be used together. reject. sig_id <sid>. sig_id <sid>. This means the match statistics are maintained for each unique source IP address. \ 103
. An event filter may be used to manage number of alerts after the rule action is enabled by rate filter. or by rule. \ type <limit|threshold|both>. destination IP address. seconds <s> threshold \ gen_id <gid>. revert to the original rule action after t seconds. If t is 0. Format event_filter \ gen_id <gid>. source and destination means client and server respectively. or they are aggregated at rule level. Note that events are generated during the timeout period. • threshold Alerts every m times we see this event during the time interval. For rules related to Stream5 sessions. then ignores events for the rest of the time interval. new action replaces rule action for t seconds. then rule action is never reverted back. 0 seconds means count is a total count instead of a speciﬁc rate. rate filter may be used to detect if the number of connections to a speciﬁc server exceed a speciﬁc count.
2. for each unique destination IP address. \ type <limit|threshold|both>.2 Event Filtering
Event ﬁltering can be used to reduce the number of logged alerts for noisy rules by limiting the number of times a particular event is logged during a speciﬁed time interval. track by rule and apply to may not be used together.

seconds 60 Limit logging to every 3rd event: event_filter \ gen_id 1. seconds <s> threshold is an alias for event filter. then ignores any additional events during the time interval. Standard ﬁltering tests are applied ﬁrst. Thresholds in a rule (deprecated) will override a global event filter. time period over which count is accrued.
track by src|by dst
count c seconds s
! △NOTE filter may be deﬁned for a given gen id. gen id 0. sig_id 1852.
! △NOTE can be used to suppress excessive rate filter alerts. threshold is deprecated and will not be supported in future releases. number of rule matching in s seconds that will cause event filter limit to be exceeded. If more than one event filter is Only one event
applied to a speciﬁc gen id. c must be nonzero value. sig id.all are required. If gen id is also 0. or for each unique destination IP addresses. Specify the signature ID of an associated rule. Type threshold alerts every m times we see this event during the time interval. Global event filters do not override what’s in a signature or a more speciﬁc stand-alone event filter. rate is tracked either by source IP address. sig id != 0 is not allowed). then ignores events for the rest of the time interval. or destination IP address. sig id 0 speciﬁes a ”global” ﬁlter because it applies to all sig ids for the given gen id. sig id pair. track by_src. count 1. the ﬁrst new action event event filters
of the timeout period is never suppressed. sig_id 1851. (gen id 0. Examples Limit logging to 1 event per 60 seconds: event_filter \ gen_id 1. the global ﬁltering test is applied. \ type limit. Both formats are equivalent and support the options described below . Type both alerts once per time interval after seeing m occurrences of the event. if they do not block an event from being logged. Such events indicate a change of state that are signiﬁcant to the user monitoring the network. This means count is maintained for each unique source IP addresses. however. event filters with sig id 0 are considered ”global” because they apply to all rules with the given gen id. sig id 0 can be used to specify a ”global” threshold that applies to all rules. seconds 60 104
\
. Option gen id <gid> sig id <sid> type limit|threshold|both Description Specify the generator ID of an associated rule.count <c>. Snort will terminate with an error while reading the conﬁguration information. then the ﬁlter applies to all rules. \ count 3. type limit alerts on the 1st m events during the time interval. \ type threshold. s must be nonzero value. track by_src. Ports or anything else are not tracked.

\ count 1. \ suppress \ gen_id <gid>. You may apply multiple suppressions to a non-zero SID. seconds 60 Limit to logging 1 event per 60 seconds per IP. sig_id 0. \ track <by_src|by_dst>.map for details on gen ids. Format The suppress conﬁguration has two forms: suppress \ gen_id <gid>. sig_id 1853. Suppression are standalone conﬁgurations that reference generators. Read genmsg. This allows a rule to be completely suppressed. sig_id 0. ip <ip-list>
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. track by_src. \ type limit. Suppression uses an IP list to select speciﬁc networks and users for suppression.3 Event Suppression
Event suppression stops speciﬁed events from ﬁring without removing the rule from the rule base. Users can also conﬁgure a memcap for threshold with a “conﬁg:” option: config event_filter: memcap <bytes> # this is deprecated: config threshold: memcap <bytes>
2. and IP addresses via an IP list . sig_id <sid>.Limit logging to just 1 event per 60 seconds. seconds 60 Events in Snort are generated in the usual way. \ count 30. but only if we exceed 30 events in 60 seconds: event_filter \ gen_id 1. SIDs. seconds 60 Limit to logging 1 event per 60 seconds per IP triggering each rule (rule gen id is 1): event_filter \ gen_id 1. sig_id <sid>. track by_src. event ﬁlters are handled as part of the output system. triggering each rule for each event generator: event_filter \ gen_id 0. track by_src. or suppressed when the causative trafﬁc is going to or coming from a speciﬁc IP or group of IP addresses. \ type both. \ type limit. You may also combine one event filter and several suppressions to the same non-zero SID. Suppression tests are performed prior to either standard or global thresholding tests. \ count 1.4.

but if present. sig_id 1852.0/24
2.4 Event Logging
Snort supports logging multiple events per packet/stream that are prioritized with different insertion methods. sig_id 1852: Suppress this event from this IP: suppress gen_id 1. order events This argument determines the way that the incoming events are ordered. You can’t log more than the max event number that was speciﬁed. If track is provided. Restrict the suppression to only source or destination IP addresses (indicated by track parameter) determined by ¡list¿. We currently have two different methods: • priority . ip 10. The default value is 8. ip 10. sig id 0 speciﬁes a ”global” ﬁlter because it applies to all sig ids for the given gen id. ip must be provided as well. ip must be provided as well. The default value is 3. gen id 0.1.4.
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. 3. only 8 events will be stored for a single packet or stream.Option gen id <gid> sig id <sid> track by src|by dst ip <list>
Description Specify the generator ID of an associated rule.1. For example. This is optional. sig_id 1852.54 Suppress this event to this CIDR block: suppress gen_id 1.1. if the event queue has a max size of 8. sig id 0 can be used to specify a ”global” threshold that applies to all rules. track by_dst. The general conﬁguration of the event queue is as follows: config event_queue: [max_queue [size]] [log [size]] [order_events [TYPE]] Event Queue Conﬁguration Options There are three conﬁguration options to the conﬁguration parameter ’event queue’. Suppress by source IP address or destination IP address.The highest priority (1 being the highest) events are ordered ﬁrst. log This determines the number of events to log for a given packet or stream. max queue This determines the maximum size of the event queue. Specify the signature ID of an associated rule.
Examples Suppress this event completely: suppress gen_id 1. such as max content length or event ordering using the event queue. 1.1. track by_src. 2.

When a ﬁle name is provided in profile rules or profile preprocs. The default value is content length.5 Performance Proﬁling
Snort can provide statistics on rule and preprocessor performance. If append is not speciﬁed.conf and Snort will print statistics on the worst (or all) performers on exit. These ﬁles will be found in the logging directory. but change event order: config event_queue: order_events priority Use the default event queue values but change the number of logged events: config event_queue: log 2
2. The ﬁlenames will have timestamps appended to them.filename <filename> [append]] • <num> is the number of rules to print • <sort option> is one of: checks matches nomatches avg ticks avg ticks per match avg ticks per nomatch total ticks • <filename> is the output ﬁlename • [append] dictates that the output will go to the same ﬁle each time (optional) 107
. \ sort <sort_option> \ [.1 Rule Proﬁling
Format config profile_rules: \ print [all | <num>]. Event Queue Conﬁguration Examples The default conﬁguration: config event_queue: max_queue 8 log 3 order_events content_length Example of a reconﬁgured event queue: config event_queue: max_queue 10 log 3 order_events content_length Use the default event queue values. and rules that have a longer content are ordered before rules with shorter contents. etc. log.Rules are ordered before decode or preprocessor alerts.• content length . To use this feature. The method in which events are ordered does not affect rule types such as pass.5.
2. a new ﬁle will be created each time Snort is run. Each require only a simple config option to snort. you must build snort with the --enable-perfprofiling option to the conﬁgure script. the statistics will be saved in these ﬁles. alert.

• Avg Ticks per Nonmatch Interpreting this info is the key. The Microsecs (or Ticks) column is important because that is the total time spent evaluating a given rule. But, if that rule is causing alerts, it makes sense to leave it alone. A high Avg/Check is a poor performing rule, that most likely contains PCRE. High Checks and low Avg/Check is usually an any->any rule with few rule options and no content. Quick to check, the few options may or may not match. We are looking at moving some of these into code, especially those with low SIDs. By default, this information will be printed to the console when Snort exits. You can use the ”ﬁlename” option in snort.conf to specify a ﬁle where this will be written. If ”append” is not speciﬁed, a new ﬁle will be created each time Snort is run. The ﬁlenames will have timestamps appended to them. These ﬁles will be found in the logging directory.

• Print the top 10 preprocessors, based on highest average time config profile preprocs: print 10, sort avg ticks

• Print all preprocessors, sorted by number of checks config profile preprocs: Output Snort will print a table much like the following at exit. Conﬁguration line used to print the above table: config profile_rules: \ print 3, sort total_ticks print all, sort checks

The columns represent: • Number (rank) - The number is indented for each layer. Layer 1 preprocessors are listed under their respective caller (and sorted similarly). • Preprocessor Name • Layer - When printing a speciﬁc number of preprocessors all subtasks info for a particular preprocessor is printed for each layer 0 preprocessor stat. • Checks (number of times preprocessor decided to look at a packet, ports matched, app layer header was correct, etc) • Exits (number of corresponding exits – just to verify code is instrumented correctly, should ALWAYS match Checks, unless an exception was trapped) • CPU Ticks • Avg Ticks per Check • Percent of caller - For non layer 0 preprocessors, i.e. subroutines within preprocessors, this identiﬁes the percent of the caller’s ticks that is spent for this subtask. Because of task swapping, non-instrumented code, and other factors, the Pct of Caller ﬁeld will not add up to 100% of the caller’s time. It does give a reasonable indication of how much relative time is spent within each subtask. By default, this information will be printed to the console when Snort exits. You can use the ”ﬁlename” option in snort.conf to specify a ﬁle where this will be written. If ”append” is not speciﬁed, a new ﬁle will be created each time Snort is run. The ﬁlenames will have timestamps appended to them. These ﬁles will be found in the logging directory.

2.5.3 Packet Performance Monitoring (PPM)
PPM provides thresholding mechanisms that can be used to provide a basic level of latency control for snort. It does not provide a hard and fast latency guarantee but should in effect provide a good average latency control. Both rules and packets can be checked for latency. The action taken upon detection of excessive latency is conﬁgurable. The following sections describe conﬁguration, sample output, and some implementation details worth noting. To use PPM, you must build with the –enable-ppm or the –enable-sourceﬁre option to conﬁgure. PPM is conﬁgured as follows: # Packet configuration: config ppm: max-pkt-time <micro-secs>, \ fastpath-expensive-packets, \ pkt-log, \ debug-pkts # Rule configuration: config ppm: max-rule-time <micro-secs>, \ threshold count, \ suspend-expensive-rules, \ suspend-timeout <seconds>, \ rule-log [log] [alert] Packets and rules can be conﬁgured separately, as above, or together in just one conﬁg ppm statement. Packet and rule monitoring is independent, so one or both or neither may be enabled.

Therefore this implementation cannot implement a precise latency guarantee with strict timing guarantees.6 Output Modules
Output modules are new as of version 1. Hence the reason this is considered a best effort approach. Latency control is not enforced after each preprocessor. Due to the granularity of the timing measurements any individual packet may exceed the user speciﬁed packet or rule processing time limit.. PPM: Process-BeginPkt[61] caplen=60 PPM: Pkt[61] Used= 8.Sample Snort Run-time Output . not processor usage by Snort. • Time checks are made based on the total system time. 1 nc-rules tested. packet fastpathed. after 114
.. This was a conscious design decision because when a system is loaded. not just the processor time the Snort application receives.21764 usecs PPM: Process-EndPkt[64] .3659 usecs PPM: Process-EndPkt[62] PPM: PPM: PPM: PPM: Pkt-Event Pkt[63] used=56. Process-BeginPkt[63] caplen=60 Pkt[63] Used= 8. it is recommended that you tune your thresholding to operate optimally when your system is under load.633125 usecs Rule Performance Summary: max rule time : 50 usecs rule events : 0 avg nc-rule time : 0. latency thresholding is presently only available on Intel and PPC platforms. The output modules are run when the alert or logging subsystems of Snort are called.. the latency for a packet is based on the total system time. • Since this implementation depends on hardware based high performance frequency counters.394 usecs Process-EndPkt[63]
PPM: Process-BeginPkt[64] caplen=60 PPM: Pkt[64] Used= 8..2675 usecs Implementation Details • Enforcement of packet and rule processing times is done after processing each rule. Sample Snort Exit Output Packet Performance Summary: max packet time : 50 usecs packet events : 1 avg pkt time : 0.6.
2. Therefore. • This implementation is software based and does not use an interrupt driven timing mechanism and is therefore subject to the granularity of the software based timing tests. They allow Snort to be much more ﬂexible in the formatting and presentation of output to its users.15385 usecs PPM: Process-EndPkt[61] PPM: Process-BeginPkt[62] caplen=342 PPM: Pkt[62] Used= 65.0438 usecs. 0 rules.

Format output alert_fast: [<filename> ["packet"] [<limit>]] <limit> ::= <number>[(’G’|’M’|K’)] • filename: the name of the log ﬁle. output alert_syslog: \ [host=<hostname[:<port>].1:514. By default.fast
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.6. <facility> <priority> <options>
2. The minimum is 1 KB. The default host is 127.1.6. • packet: this option will cause multiline entries with full packet headers to be logged. only brief single-line entries are logged. Example output alert_fast: alert.0. The default port is 514. • limit: an optional limit on ﬁle size which defaults to 128 MB.] \ <facility> <priority> <options>
Example output alert_syslog: host=10. You may specify ”stdout” for terminal output. See 2.0. The default name is ¡logdir¿/alert.1.Options • log cons • log ndelay • log perror • log pid Format alert_syslog: \ <facility> <priority> <options>
! △NOTE
As WIN32 does not run syslog servers locally by default.1.2 alert fast
This will print Snort alerts in a quick one-line format to a speciﬁed output ﬁle. a hostname and port can be passed as options. It is a faster alerting method than full alerts because it doesn’t need to print all of the packet headers to the output ﬁle and because it logs to only 1 ﬁle.13 for more information. The name may include an absolute or relative path.

6. External programs/processes can listen in on this socket and receive Snort alert and packet data in real time.6. The default name is ¡logdir¿/alert.3 alert full
This will print Snort alert messages with full packet headers. Format output alert_full: [<filename> [<limit>]] <limit> ::= <number>[(’G’|’M’|K’)] • filename: the name of the log ﬁle.13 for more information. See 2.4 alert unixsock
Sets up a UNIX domain socket and sends alert reports to it.log.
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. Format output log_tcpdump: [<filename> [<limit>]] <limit> ::= <number>[(’G’|’M’|K’)] • filename: the name of the log ﬁle.6. When a sequence of packets is to be logged.5 log tcpdump
The log tcpdump module logs packets to a tcpdump-formatted ﬁle.13 for more information. The default name is ¡logdir¿/snort. These ﬁles will be decoded packet dumps of the packets that triggered the alerts. • limit: an optional limit on ﬁle size which defaults to 128 MB.full
2. Format alert_unixsock Example output alert_unixsock
2. The alerts will be written in the default logging directory (/var/log/snort) or in the logging directory speciﬁed at the command line. See 2. This is currently an experimental interface. The minimum is 1 KB. Example output alert_full: alert. a directory will be created per IP.6. • limit: an optional limit on ﬁle size which defaults to 128 MB.2. The creation of these ﬁles slows Snort down considerably. The name may include an absolute or relative path.6. This is useful for performing post-process analysis on collected trafﬁc with the vast number of tools that are available for examining tcpdump-formatted ﬁles. Inside the logging directory. the aggregate size is used to test the rollover condition. A UNIX timestamp is appended to the ﬁlename. You may specify ”stdout” for terminal output. The name may include an absolute or relative path. This output method is discouraged for all but the lightest trafﬁc situations.

’. or socket ﬁlename extension for UNIX-domain connections.Password used if the database demands password authentication sensor name .Database name user .Represent binary data as a base64 string.2x the size of the binary Searchability .6.Because the packet payload and option data is binary. The arguments to this plugin are the name of the database to be logged to and a parameter list.How much detailed data do you want to store? The options are: full (default) .not readable unless you are a true geek.log
2. Blobs are not used because they are not portable across databases.Represent binary data as an ASCII string. Storage requirements . it will connect using a local UNIX domain socket. Without a host name. requires post processing base64 .<.Example output log_tcpdump: snort. <database type>.very good Human readability . If a non-zero-length string is speciﬁed. Format database: <log | alert>. Storage requirements . More information on installing and conﬁguring this module can be found on the [91]incident. So i leave the encoding option to you.not readable requires post processing ascii . see Figure 2. If you choose this option. Each has its own advantages and disadvantages: hex (default) . TCP/IP communication is used. <parameter list> The following parameters are available: host . Storage requirements . Non-ASCII Data is represented as a ‘.slightly larger than the binary because some characters are escaped (&. there is no one simple and portable way to store it in a database.3 for example usage.org web page.Database username for authentication password .Specify your own name for this Snort sensor.>) Searchability . dbname . This is the only option where you will actually lose data. Parameters are speciﬁed with the format parameter = argument.very good for searching for a text string impossible if you want to search for binary human readability .3x the size of the binary Searchability .very good detail . You can choose from the following options. If you do not specify a name. then data for IP and TCP options will still be represented as hex because it does not make any sense for that data to be ASCII. port .∼1. one will be generated automatically encoding .Port number to connect to at the server host.Host to connect to.Represent binary data as a hex string.Log all details of a packet that caused an alert (including IP/TCP options and the payload)
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.impossible without post processing Human readability .6 database
This module from Jed Pickel sends Snort data to a variety of SQL databases.

The following ﬁelds are logged: timestamp. oracle. • format: The list of formatting options is below.3: Database Output Plugin Conﬁguration fast . destination port. signature. Setting the type to log attaches the database logging functionality to the log facility within the program. the output is in the order of the formatting options listed. dbname=snort user=snort host=localhost password=xyz Figure 2. There are two logging types available.5 for more details. The name may include an absolute or relative path. These are mssql.. Format output alert_csv: [<filename> [<format> [<limit>]]] <format> ::= "default"|<list> <list> ::= <field>(.output database: \ log. There are ﬁve database types available in the current version of the plugin.csv. Set the type to match the database you are using.7. <limit> ::= <number>[(’G’|’M’|K’)] • filename: the name of the log ﬁle. and protocol) Furthermore. mysql. source ip. tcp flags. mysql. If the formatting option is ”default”. If you set the type to log. destination ip. The output ﬁelds and their order may be customized. – timestamp – sig generator – sig id – sig rev – msg – proto – src – srcport – dst
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.
2. Setting the type to alert attaches the plugin to the alert output chain within the program.6. source port.
! △NOTE
The database output plugin does not have the ability to handle alerts that are generated by using the tag keyword. See section 3. there is a logging method and database type that must be deﬁned. You severely limit the potential of some analysis applications if you choose this option. The default name is ¡logdir¿/alert.Log only a minimum amount of data. postgresql.7 csv
The csv output plugin allows alert data to be written in a format easily importable to a database.<field>)* <field> ::= "dst"|"src"|"ttl" . but this is still the best choice for some applications. You may specify ”stdout” for terminal output. and odbc. the plugin will be called on the log output chain. log and alert..

snort must be built with the –enable-prelude argument passed to .or Format output alert_prelude: \
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. nostamp.log. See section 2. limit 128.prelude-ids.
! △NOTE
By default. packet logging.
Format output alert_unified2: \ filename <base filename> [.log. For more information on Prelude. To use alert prelude. uniﬁed ﬁle. mpls_event_types
2. Likewise. limit 128 output log_unified: snort.Example output alert_unified: snort. simply specify unified2. If option mpls event types is not used. nostamp] [. or true uniﬁed logging. but a slightly different logging format.6. <limit <size in MB>] [. alert logging. limit 128. <limit <size in MB>] [. then MPLS labels will be not be included in uniﬁed2 events./conﬁgure.alert. Uniﬁed2 can work in one of three modes.6. It has the same performance characteristics. MPLS labels can be included in uniﬁed2 events.9 uniﬁed 2
The uniﬁed2 output plugin is a replacement for the uniﬁed output plugin. nostamp log_unified2: filename snort. limit 128.log.log. Packet logging includes a capture of the entire packet and is speciﬁed with log unified2. see http://www.
The alert prelude output plugin is used to log to a Prelude database. uniﬁed 2 ﬁles have the ﬁle creation time (in Unix Epoch format) appended to each ﬁle when it is created. limit 128
2. nostamp] output unified2: \ filename <base file name> [.alert. mpls_event_types] output log_unified2: \ filename <base filename> [.6. <limit <size in MB>] [. mpls_event_types] Example output output output output alert_unified2: filename snort. When MPLS support is turned on. alert logging will only log events and is speciﬁed with alert unified2. limit 128. Use option mpls event types to enable this. nostamp] [. nostamp unified2: filename merged. To include both logging styles in a single.8 on uniﬁed logging for more information. nostamp unified2: filename merged.10 alert prelude
! △NOTE
support to use alert prelude is not built in by default.

one of ”sha1”. the log null plugin was introduced. ”md5” or ”cleartext”.
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.6. see http://www.com/. secrettype .Secret type./conﬁgure. To use alert aruba action.11 log null
Sometimes it is useful to be able to create rules that will alert to certain types of trafﬁc but will not cause packet log entries.2. This allows Snort to take action against users on the Aruba controller to control their network privilege levels. In Snort 1. Format output log_null Example output log_null # like using snort -n
ruletype info { type alert output alert_fast: info.
Communicates with an Aruba Networks wireless mobility controller to change the status of authenticated users.alert output log_null }
2. snort must be built with the –enable-aruba argument passed to .profile=<name of prelude profile> \ [ info=<priority number for info priority alerts>] \ [ low=<priority number for low priority alerts>] \ [ medium=<priority number for medium priority alerts>] Example output alert_prelude: profile=snort info=4 low=3 medium=2
2.arubanetworks. Format output alert_aruba_action: \ <controller address> <secrettype> <secret> <action> The following parameters are required: controller address . This is equivalent to using the -n command line option but it is able to work within a ruletype.Aruba mobility controller address.12 alert aruba action
! △NOTE
Support to use alert aruba action is not built in by default.6. For more information on Aruba Networks access control.8.

setrole:rolename .
2.8.7 Host Attribute Table
Starting with version 2.Blacklist the station by disabling all radio communication. if applicable. If the rule doesn’t have protocol metadata. Snort associates a given packet with its attribute data from the table.Action to apply to the source IP address of the trafﬁc generating an alert.6 cleartext foobar setrole:quarantine_role
2. This information is stored in an attribute table. The table is re-read during run time upon receipt of signal number 30.1 Conﬁguration Format
attribute_table filename <path to file>
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. and log tcpdump.3. For rule evaluation.9. the current log is closed and a new log is opened with a UNIX timestamp appended to the conﬁgured log name.
! △NOTE
To use a host attribute table.2. alert full. limit will be exceeded. alert csv.1) and TCP Stream reassembly policies (see section 2. Those limits are described in the respective sections. Limits are conﬁgured as follows: <limit> ::= <number>[(<gb>|<mb>|<kb>)] <gb> ::= ’G’|’g’ <mb> ::= ’M’|’m’ <kb> ::= ’K’|’k’ Rollover will occur at most once per second so if limit is too small for logging rate. or a cleartext password. action .secret . which is loaded at startup.Authentication secret conﬁgured on the Aruba mobility controller with the ”aaa xml-api client” conﬁguration command. blacklist . or the trafﬁc doesn’t have any matching service information. When a conﬁgured limit is reached. s Example output alert_aruba_action: \ 10. represented as a sha1 or md5 hash.2.Change the user´ role to the speciﬁed rolename.13 Log Limits
This section pertains to logs produced by alert fast.2).
2. Snort must be conﬁgured with the –enable-targetbased ﬂag.1. unified and unified2 also may be given limits. Snort has the capability to use information from an outside source to determine both the protocol for use with Snort rules. the rule relies on the port information.7.6. Rollover works correctly if snort is stopped/restarted. and IP-Frag policy (see section 2. service information is used instead of the ports when the protocol metadata in the rule matches the service corresponding to the trafﬁc.

8.0</ATTRIBUTE_VALUE> <CONFIDENCE>89</CONFIDENCE> </VERSION> </APPLICATION> </CLIENT> </CLIENTS> </HOST> </ATTRIBUTE_TABLE> </SNORT_ATTRIBUTES>
! △NOTE2. ssh. The application and version for a given service attribute.</VERSION> </APPLICATION> </SERVICE> <SERVICE> <PORT> <ATTRIBUTE_VALUE>2300</ATTRIBUTE_VALUE> <CONFIDENCE>100</CONFIDENCE> </PORT> <IPPROTO> <ATTRIBUTE_VALUE>tcp</ATTRIBUTE_VALUE> <CONFIDENCE>100</CONFIDENCE> </IPPROTO> <PROTOCOL> <ATTRIBUTE_VALUE>telnet</ATTRIBUTE_VALUE> <CONFIDENCE>100</CONFIDENCE> </PROTOCOL> <APPLICATION> <ATTRIBUTE_VALUE>telnet</ATTRIBUTE_VALUE> <CONFIDENCE>50</CONFIDENCE> </APPLICATION> </SERVICE> </SERVICES> <CLIENTS> <CLIENT> <IPPROTO> <ATTRIBUTE_VALUE>tcp</ATTRIBUTE_VALUE> <CONFIDENCE>100</CONFIDENCE> </IPPROTO> <PROTOCOL> <ATTRIBUTE_VALUE>http</ATTRIBUTE_VALUE> <CONFIDENCE>91</CONFIDENCE> </PROTOCOL> <APPLICATION> <ATTRIBUTE_VALUE>IE Http Browser</ATTRIBUTE_VALUE> <CONFIDENCE>90</CONFIDENCE> <VERSION> <ATTRIBUTE_VALUE>6. udp. The conﬁdence metric may be used to indicate the validity of a given service or client application and its respective elements. That ﬁeld is not currently used by Snort. 125
. and any client attributes are ignored. They will be used in a future release. and protocol (http. etc) are used. Of the service With Snort
attributes. the stream and IP frag information are both used. port. etc).1. only the IP protocol (tcp. but may be in future releases. A DTD for veriﬁcation of the Host Attribute Table XML ﬁle is provided with the snort packages. for a given host entry.

3 Attribute Table Example
In the example above.168. On that host. Telnet. a host running Red Hat 2. The IP stack fragmentation and stream reassembly is mimicked by the ”linux” conﬁguration (see sections 2. Snort will inspect packets for a connection to 192. even if the telnet portion of the FTP/Telnet preprocessor is only conﬁgured to inspect port 23.234 port 2300 because it is identiﬁed as telnet.168.1. • Alert: Rule Has Service Metadata. HTTP Inspect will NOT process the packets on a connection to 192. The following few scenarios identify whether a rule will be inspected or not. This host has an IP address of 192. rules conﬁgured for speciﬁc ports that have a service metadata will be processed based on the service identiﬁed by the attribute table.established.) 126
. TCP port 22 is ssh (running Open SSH). sid:10000001. http dcerpc dns imap ftp netbios-dgm isakmp pop2 ftp-data netbios-ns mysql pop3 telnet netbios-ssn oracle snmp smtp nntp cvs ssh ﬁnger shell tftp sunrpc x11
Attribute Table Affect on rules Similar to the application layer preprocessors. flow:to_server. Attribute Table Affect on preprocessors • Network Layer Preprocessors Each of the network layer preprocessors (frag3 and stream5) make use of the respective FRAG POLICY and STREAM POLICY in terms of how data is handled for reassembly for packets being received by that host. Conversely. metadata: service telnet. For example. Connection Service Matches One of them The following rule will be inspected and alert on trafﬁc to host 192.) • Alert: Rule Has Multiple Service Metadata.6 is described.2. Snort will ONLY inspect the rules that have the service that matches the connection. for example. metadata: service telnet. FTP.1 and 2. sid:10000002. Snort uses the service rather than the port.established.168.234 port 2300 as telnet.2).168. service smtp.1.7.234 port 2300 because it is identiﬁed as telnet. alert tcp any any -> any 23 (msg:"Telnet traffic". flow:to_server.234 port 2300 because it is identiﬁed as telnet. HTTP Inspect is conﬁgured to inspect trafﬁc on port 2300. SMTP.2.1.1. if. and TCP port 2300 is telnet. Below is a list of the common services used by Snort’s application layer preprocessors and Snort rules (see below). If there are rules that use the service and other rules that do not but the port matches. etc) make use of the SERVICE information for connections destined to that host on that port. alert tcp any any -> any 23 (msg:"Telnet traffic". When both service metadata is present in the rule and in the connection. • Application Layer Preprocessors The application layer preprocessors (HTTP.1.2. Connection Service Matches The following rule will be inspected and alert on trafﬁc to host 192.234.168.

168. Connection Service Does Not Match.8. flow:to_server.234 port 2300 because that trafﬁc is identiﬁed as telnet.) • No Alert: Rule Has No Service Metadata. sid:10000003.• No Alert: Rule Has Service Metadata.established.2 Directives
Syntax dynamicpreprocessor [ file <shared library path> | directory <directory of shared libraries> ] Description Tells snort to load the dynamic preprocessor shared library (if ﬁle is used) or all dynamic preprocessor shared libraries (if directory is used). flow:to_server.
. alert tcp any any -> any 23 (msg:"Port 23 traffic". followed by the full or relative path to a directory of preprocessor shared libraries. Port Matches The following rule will be inspected and alert on trafﬁc to host 192.168.6. Or.established. Port Matches The following rule will NOT be inspected and NOT alert on trafﬁc to host 192. flow:to_server. sid:10000006.168.8 Dynamic Modules
Dynamically loadable modules were introduced with Snort 2. Specify file.1. sid:10000007. alert tcp any any -> any 2300 (msg:"Port 2300 traffic". specify directory.1.established.established. metadata: service ssh. alert tcp any any -> any 2300 (msg:"Port 2300 traffic".
2. but the service is ssh.
! △NOTE
To disable use of dynamic modules.) • Alert: Rule Has No Service Metadata.) • Alert: Rule Has No Service Metadata. metadata: service telnet.) alert tcp any any -> any 2300 (msg:"Port 2300 traffic".234 port 2300 because the port matches.1.8. Snort must be conﬁgured with the --disable-dynamicplugin ﬂag. Packet has service + other rules with service The ﬁrst rule will NOT be inspected and NOT alert on trafﬁc to host 192.168. They can be loaded via directives in snort. sid:10000005.1 Format
<directive> <parameters>
2. See chapter 4 for more 127 information on dynamic preprocessor libraries. (Same effect as --dynamic-preprocessor-lib or --dynamic-preprocessor-lib-dir options). alert tcp any any -> any 2300 (msg:"SSH traffic".1.234 port 2300 because the port does not match.)
2. flow:to_server. sid:10000004. Port Does Not Match The following rule will NOT be inspected and NOT alert on trafﬁc to host 192.conf or via command-line options.established.234 port 2300 because the service is identiﬁed as telnet and there are other rules with that service. flow:to_server. followed by the full or relative path to the shared library.

This option is enabled by default and the behavior is for Snort to restart if any nonreloadable options are added/modiﬁed/removed. Specify file. however. followed by the full or relative path to the shared library.
! △NOTE
This functionality is not currently supported in Windows.2 Reloading a conﬁguration
First modify your snort. Specify file. send Snort a SIGHUP signal. Snort will restart (as it always has) upon receipt of a SIGHUP. All newly created sessions will. the main Snort packet processing thread will swap in the new conﬁguration to use and will continue processing under the new conﬁguration. There is also an ancillary option that determines how Snort should behave if any non-reloadable options are changed (see section 2.
2.9. $ kill -SIGHUP <snort pid>
! △NOTE
If reload support is not enabled. specify directory. Tells snort to load the dynamic detection rules shared library (if ﬁle is used) or all dynamic detection rules shared libraries (if directory is used). (Same effect as --dynamic-detection-lib or --dynamic-detection-lib-dir options).conf (the ﬁle passed to the -c option on the command line). Or.3 below). to initiate a reload.1 Enabling support
To enable support for reloading a conﬁguration. Then.9 Reloading a Snort Conﬁguration
Snort now supports reloading a conﬁguration in lieu of restarting Snort in so as to provide seamless trafﬁc inspection during a conﬁguration change. add --enable-reload to conﬁgure when compiling. add --disable-reload-error-restart in addition to --enable-reload to conﬁgure when compiling. Note that for some preprocessors. Or. When a swappable conﬁguration object is ready for use.
128
. See chapter 4 for more information on dynamic engine libraries. use the new conﬁguration.9. (Same effect as --dynamic-engine-lib or --dynamic-preprocessor-lib-dir options). followed by the full or relative path to a directory of detection rules shared libraries.dynamicengine [ file <shared library path> | directory <directory of shared libraries> ]
dynamicdetection [ file <shared library path> | directory <directory of shared libraries> ]
Tells snort to load the dynamic engine shared library (if ﬁle is used) or all dynamic engine shared libraries (if directory is used). existing session data will continue to use the conﬁguration under which they were created in order to continue with proper state for that session.g. e.
2. See chapter 4 for more information on dynamic detection rules libraries. followed by the full or relative path to the shared library. To disable this behavior and have Snort exit instead of restart. specify directory.9.
2. followed by the full or relative path to a directory of preprocessor shared libraries. A separate thread will parse and create a swappable conﬁguration object while the main Snort packet processing thread continues inspecting trafﬁc under the current conﬁguration.

Modifying any of these options will cause Snort to restart (as a SIGHUP previously did) or exit (if --disable-reload-error-restart was used to conﬁgure Snort). dynamicengine and dynamicpreprocessor are not reloadable.conf -T
2. i. Changes to the following options are not reloadable: attribute_table config alertfile config asn1 config chroot config daemon config detection_filter config flowbits_size config interface config logdir config max_attribute_hosts config nolog config no_promisc config pkt_count config rate_filter config read_bin_file config response config set_gid config set_uid config snaplen config threshold dynamicdetection dynamicengine dynamicpreprocessor output In certain cases.3 Non-reloadable conﬁguration options
There are a number of option changes that are currently non-reloadable because they require changes to output. e.e.9. only some of the parameters to a conﬁg option or preprocessor conﬁguration are not reloadable. startup memory allocations. config ppm: max-rule-time <int> rule-log config profile_rules 129
. etc. • Adding/modifying/removing preprocessor conﬁgurations are reloadable (except as noted below). Non-reloadable conﬁguration options of note: • Adding/modifying/removing shared objects via dynamicdetection. Reloadable conﬁguration options of note: • Adding/modifying/removing text rules and variables are reloadable. • Any changes to output will require a restart. Those parameters are listed below the relevant conﬁg option or preprocessor. $ snort -c snort.! △NOTEconﬁguration will still result in Snort fatal erroring. so you should test your new conﬁguration An invalid
before issuing a reload.g. any new/modiﬁed/removed shared objects will require a restart.

This will allow administrators to specify multiple snort conﬁguration ﬁles and bind each conﬁguration to one or more VLANs or subnets rather than running one Snort for each conﬁguration required. Negative vland Ids and alphanumeric are not supported. Valid vlanId is any number in 0-4095 range.1 Creating Multiple Conﬁgurations
Default conﬁguration for snort is speciﬁed using the existing -c option.Refers to ip subnets.filename print sort config profile_preprocs filename print sort preprocessor dcerpc2 memcap preprocessor frag3_global max_frags memcap prealloc_frags prealloc_memcap disabled preprocessor perfmonitor file snortfile preprocessor sfportscan memcap logfile disabled preprocessor stream5_global memcap max_tcp max_udp max_icmp track_tcp track_udp track_icmp
2.conf> vlan <vlanIdList> config binding: <path_to_snort. vlanIdList . ipList . 130
. using the following conﬁguration line: config binding: <path_to_snort. Spaces are allowed within ranges. Subnets can be CIDR blocks for IPV6 or IPv4. A default conﬁguration binds multiple vlans or networks to non-default conﬁgurations.Refers to the absolute or relative path to the snort. The format for ranges is two vlanId separated by a ”-”.conf> net <ipList> path to snort.conf .Refers to the comma seperated list of vlandIds and vlanId ranges.10.
2. VLANs/Subnets not bound to any speciﬁc conﬁguration will use the default conﬁguration.conf for speciﬁc conﬁguration. A maximum of 512 individual IPv4 or IPv6 addresses or CIDRs can be speciﬁed. Each unique snort conﬁguration ﬁle will create a new conﬁguration instance within snort. Each conﬁguration can have different preprocessor settings and detection rules.10 Multiple Conﬁgurations
Snort now supports multiple conﬁgurations based on VLAN Id or IP subnet within a single instance of Snort.

Even though
2. The following conﬁg options are speciﬁc to each conﬁguration. 131 checksum_drop disable_decode_alerts disable_decode_drops disable_ipopt_alerts disable_ipopt_drops disable_tcpopt_alerts disable_tcpopt_drops disable_tcpopt_experimental_alerts disable_tcpopt_experimental_drops disable_tcpopt_obsolete_alerts disable_tcpopt_obsolete_drops disable_ttcp_alerts disable_tcpopt_ttcp_alerts disable_ttcp_drops
. A rule shares all parts of the rule options. policy_id policy_mode policy_version The following conﬁg options are speciﬁc to each conﬁguration. they are included as valid in terms of conﬁguring Snort. If a rule is not speciﬁed in a conﬁguration then the rule will never raise an event for the conﬁguration. payload detection options. including the general options. If the rules in a conﬁguration use variables.! △NOTE can not be used in the same line. non-payload detection options. those variables must be deﬁned in that conﬁguration. config config config config config config config config config config config config config config Rules Rules are speciﬁc to conﬁgurations but only some parts of a rule can be customized for performance reasons.2 Conﬁguration Speciﬁc Elements
Conﬁg Options Generally conﬁg options deﬁned within the default conﬁguration are global by default i. the default values of the option (not the default conﬁguration values) take effect. Conﬁgurations can be applied based on either Vlans or Vlan and Subnets
Subnets not both. Parts of the rule header can be speciﬁed differently across conﬁgurations. Variables Variables deﬁned using ”var”. If not deﬁned in a conﬁguration.
! △NOTE Vlan Ids 0 and 4095 are reserved.10. and post-detection options. their value applies to all other conﬁgurations.e. ”portvar” and ”ipvar” are speciﬁc to conﬁgurations. limited to: Source IP address and port Destination IP address and port Action A higher revision of a rule in one conﬁguration will override other revisions of the same rule in other conﬁgurations.

If the bound conﬁguration is the default conﬁguration. that can be applied to the packet. including:
132
. The packet is assigned non-default conﬁguration if found otherwise the check is repeated using source IP address.Refers to a 16-bit unsigned value.10.3 How Conﬁguration is applied?
Snort assigns every incoming packet to a unique conﬁguration based on the following criteria. For addressed based conﬁguration binding. then destination IP address is searched to the most speciﬁc subnet that is bound to a non-default conﬁguration. In the end.
To enable vlanId logging in uniﬁed2 records the following option can be used. A preprocessor must be conﬁgured in default conﬁguration before it can be conﬁgured in non-default conﬁguration. Events and Output An unique policy id can be assigned by user. default conﬁguration is used if no other matching conﬁguration is found.Preprocessors Preprocessors conﬁgurations can be deﬁned within each vlan or subnet speciﬁc conﬁguration.
! △NOTE
If no policy id is speciﬁed. snort assigns 0 (zero) value to the conﬁguration. In this case.9 includes a number of changes to better handle inline operation. are processed only in default policy. then the innermost VLANID is used to ﬁnd bound conﬁguration. vlan_event_types (true unified logging) filename .
2.Refers to the absolute or relative ﬁlename. The options control total memory usage for a preprocessor across all policies. This is required as some mandatory preprocessor conﬁguration options are processed only in default conﬁguration.When this option is set. Options controlling speciﬁc preprocessor memory usage. output alert_unified2: vlan_event_types (alert logging only) output unified2: filename <filename>. snort will use uniﬁed2 event type 104 and 105 for IPv4 and IPv6 respectively.
2. If VLANID is present. These options are ignored in non-default policies without raising an error.11 Active Response
Snort 2. vlan event types . through speciﬁc limit on memory usage or number of instances.
! △NOTE
Each event logged will have the vlanId from the packet if vlan headers are present otherwise 0 will be used. to each conﬁguration using the following conﬁg line: config policy_id: <id> id . this can lead to conﬂicts between conﬁgurations if source address is bound to one conﬁguration and destination address is bound to another. snort will use the ﬁrst conﬁguration in the order of deﬁnition. This policy id will be used to identify alerts from a speciﬁc conﬁguration in the uniﬁed2 records.

/configure --enable-active-response / -DACTIVE_RESPONSE preprocessor stream5_global: \ max_active_responses <max_rsp>.25) <min_sec> ::= (1.300) Active responses will be encoded based on the triggering packet.1 Enabling Active Response
This enables active responses (snort will send TCP RST or ICMP unreachable/port) when dropping a session. if and only if attempts ¿ 0./configure --enable-flexresp / -DENABLE_RESPOND -DENABLE_RESPONSE config flexresp: attempts 1 * Flexresp2 is deleted.
2. these features are deprecated.11.11. This sequence ”straﬁng” is really only useful in passive mode.
2. At most 1 ICMP unreachable is sent. Each attempt (sent in rapid succession) has a different sequence number.20)
2. non-functional. In inline mode the reset is put straight into the stream in lieu of the triggering packet so straﬁng is not necessary./configure --enable-active-response config response: attempts <att> <att> ::= (1.. TCP data (sent for react) is multiplied similarly. . TTL will be set to the value captured at session pickup. .• a single mechanism for all responses • fully encoded reset or icmp unreachable packets • updated ﬂexible response rule option • updated react rule option • added block and sblock rule actions These changes are outlined below..2 Conﬁgure Sniping
Conﬁgure the number of attempts to land a TCP RST within the session’s current window (so that it is accepted by the receiving TCP). * Flexresp is deleted. and will be deleted in a future release:
133
. these features are no longer avaliable: ..11. \ min_response_seconds <min_sec> <max_rsp> ::= (0.3 Flexresp
Flexresp and ﬂexresp2 are replaced with ﬂexresp3. Each active response will actually cause this number of TCP resets to be sent.

The new version always sends the page to the client.5 Rule Actions
The block and sblock actions have been introduced as synonyms for drop and sdrop to help avoid confusion between packets dropped due to load (eg lack of available buffers for incoming packets) and packets blocked due to Snort’s analysis. msg:"Unauthorized Access Prohibited!". a resp option can be used instead.
135
. [proxy <port#>] The original version sent the web page to one end of the session only if the other end of the session was port 80 or the optional proxy port.
2. sid:4. <dep_opts>] These options are deprecated: <dep_opts> ::= [block|warn]. \ react: <react_opts>. The deprecated options are ignored.11.<default_msg> ::= \ "You are attempting to access a forbidden site.<br />" \ "Consult your system administrator for details.". If no page should be sent. This is an example rule: drop tcp any any -> any $HTTP_PORTS ( \ content: "d".) <react_opts> ::= [msg] [.

protocol.1.1: Sample Snort Rule 136
. In current versions of Snort. This was required in versions prior to 1.) Figure 3. and the source and destination ports information. where.1 illustrates a sample Snort rule. the rule header and the rule options. they are just used for the sake of making tighter deﬁnitions of packets to collect or alert on (or drop.
! △NOTE
Note that the rule options section is not speciﬁcally required by any rule. The ﬁrst item in a rule is the rule alert tcp any any -> 192. Snort rules are divided into two logical sections.2 Rules Headers
3. msg:"mountd access". There are a number of simple guidelines to remember when developing Snort rules that will help safeguard your sanity. the elements can be considered to form a logical AND statement. the various rules in a Snort rules library ﬁle can be considered to form a large logical OR statement. Figure 3. The rule option section contains alert messages and information on which parts of the packet should be inspected to determine if the rule action should be taken.
3.
All of the elements in that make up a rule must be true for the indicated rule action to be taken.1 Rule Actions
The rule header contains the information that deﬁnes the who.
The text up to the ﬁrst parenthesis is the rule header and the section enclosed in parenthesis contains the rule options. rules may span multiple lines by adding a backslash \ to the end of the line. and what of a packet.8.168.0/24 111 \ (content:"|00 01 86 a5|". The words before the colons in the rule options section are called option keywords. lightweight rules description language that is ﬂexible and quite powerful. When taken together. source and destination IP addresses and netmasks. Most Snort rules are written in a single line. The rule header contains the rule’s action.Chapter 3
Writing Snort Rules
3. At the same time. for that matter).2. as well as what to do in the event that a packet with all the attributes indicated in the rule should show up.1 The Basics
Snort uses a simple.

1 to 192.2.block and log the packet 7. RIP. OSPF. activate. then act as a log rule 6. alert . 137
.3 IP Addresses
The next portion of the rule header deals with the IP address and port information for a given rule. and /32 indicates a speciﬁc machine address. drop . Snort does not have a mechanism to provide host name lookup for the IP address ﬁelds in the conﬁg ﬁle.255.168.1. dynamic . log it. log . The CIDR designations give us a nice short-hand way to designate large address spaces with just a few characters. and dynamic. ICMP. reject.0/24 would signify the block of addresses from 192. 1. pass. the address/CIDR combination 192. and then send a TCP reset if the protocol is TCP or an ICMP port unreachable message if the protocol is UDP.alert and then turn on another dynamic rule 5.2. There are four protocols that Snort currently analyzes for suspicious behavior – TCP.ignore the packet 4. UDP. user=snort dbname=snort host=localhost }
3.block the packet. sdrop .2 Protocols
The next ﬁeld in a rule is the protocol. The rule action tells Snort what to do when it ﬁnds a packet that matches the rule criteria. IPX.log } This example will create a rule type that will log to syslog and a MySQL database: ruletype redalert { type alert output alert_syslog: LOG_AUTH LOG_ALERT output database: log. GRE. There are 5 available default actions in Snort. This example will create a type that will log to just tcpdump: ruletype suspicious { type log output log_tcpdump: suspicious.
3. For example.1. log. mysql. alert. reject . Any rule that used this designation for.1.block the packet but do not log it. You can also deﬁne your own rule types and associate one or more output plugins with them. The CIDR block indicates the netmask that should be applied to the rule’s address and any incoming packets that are tested against the rule. etc. The keyword any may be used to deﬁne any address.168.generate an alert using the selected alert method. The addresses are formed by a straight numeric IP address and a CIDR[3] block.action. and sdrop. In the future there may be more. A CIDR block mask of /24 indicates a Class C network. and then log the packet 2. if you are running Snort in inline mode. such as ARP. you have additional options which include drop. pass . You can then use the rule types as actions in Snort rules. the destination address would match on any address in that range. activate .remain idle until activated by an activate rule . 8.168. /16 a Class B network. In addition. and IP. IGRP. say.log the packet 3.

The negation operator may be applied against any of the other rule types (except any.168.
3. including any ports.5 The Direction Operator
The direction operator -> indicates the orientation.1.. You may also specify lists of IP addresses.
This rule’s IP addresses indicate any tcp packet with a source IP address not originating from the internal network and a destination address on the internal network. and by negation.2. how Zen.168. ranges.1.0/24 111 \ (content:"|00 01 86 a5|". and the destination address was set to match on the 192.1. Static ports are indicated by a single port number.0/24 1:1024 log udp trafﬁc coming from any port and destination ports ranging from 1 to 1024 log tcp any any -> 192.168. For the time being.1. such as 111 for portmapper. 23 for telnet.0 Class C network.0/24] 111 (content:"|00 01 86 a5|". or direction.) Figure 3.
Port negation is indicated by using the negation operator !.2. the negation operator. if for some twisted reason you wanted to log everything except the X Windows ports. such as in Figure 3. meaning literally any port. msg:"external mountd access".0/24 500: log tcp trafﬁc from privileged ports less than or equal to 1024 going to ports greater than or equal to 500 Figure 3. The range operator may be applied in a number of ways to take on different meanings.2: Example IP Address Negation Rule alert tcp ![192. For example.5. static port deﬁnitions.2.4: Port Range Examples 138
. \ msg:"external mountd access". etc. For example.0/24.1. or 80 for http.1.10.1.10.168.alert tcp !192. The negation operator is indicated with a !. Any ports are a wildcard value.168.3 for an example of an IP list in action. an easy modiﬁcation to the initial example is to make it alert on any trafﬁc that originates outside of the local net with the negation operator as shown in Figure 3.0/24 :6000 log tcp trafﬁc from any port going to ports less than or equal to 6000 log tcp any :1024 -> 192.1. the IP list may not include spaces between the addresses.3: IP Address Lists In Figure 3.168.) Figure 3. you could do something like the rule in Figure 3. Port ranges are indicated with the range operator :.0/24] any -> \ [192. An IP list is speciﬁed by enclosing a comma separated list of IP addresses and CIDR blocks within square brackets.1.0/24 any -> 192. the source IP address was set to match for any computer talking.168.0/24. which would translate to none.1.168.
3. This operator tells Snort to match any IP address except the one indicated by the listed IP address.4 Port Numbers
Port numbers may be speciﬁed in a number of ways. There is an operator that can be applied to IP addresses.1.1. The IP address and port numbers on the left side of the direction operator is considered to be the trafﬁc coming from the source log udp any any -> 192. of the trafﬁc that the rule applies to.).1.. See Figure 3.4.

1.7.7.) character. Dynamic rules act just like log rules.1. All Snort rule options are separated from each other using the semicolon (. note that there is no <.3 Rule Options
Rule options form the heart of Snort’s intrusion detection engine. activates:1.168.8. so there’s value in collecting those packets for later analysis. If the buffer overﬂow happened and was successful.0/24 23 Figure 3.0/24 !6000:6010 Figure 3.
3.operator.168. count:50. The reason the <.) Figure 3.0/24 any <> 192. count. You can now have one rule activate another when it’s action is performed for a set number of packets.6: Snort rules using the Bidirectional Operator host. combining ease of use with power and ﬂexibility. Activate rules are just like alerts but also tell Snort to add a rule when a speciﬁc network event occurs.10).6. Dynamic rules are just like log rules except are dynamically enabled when the activate rule id goes off. An example of the bidirectional operator being used to record both sides of a telnet session is shown in Figure 3. Dynamic rules have a second required ﬁeld as well. \ msg:"IMAP buffer overflow!".1. except they have a *required* option ﬁeld: activates.6. This tells Snort to consider the address/port pairs in either the source or destination orientation. and the address and port information on the right side of the operator is the destination host.
Activate/dynamic rule pairs give Snort a powerful capability.7.) dynamic tcp !$HOME_NET any -> $HOME_NET 143 (activated_by:1. such as telnet or POP3 sessions. Put ’em together and they look like Figure 3. In Snort versions before 1. which is indicated with a <> symbol. These rules tell Snort to alert when it detects an IMAP buffer overﬂow and collect the next 50 packets headed for port 143 coming from outside $HOME NET headed to $HOME NET. activate tcp !$HOME_NET any -> $HOME_NET 143 (flags:PA.5) and ﬂowbits (3.5: Example of Port Negation log tcp !192.6 Activate/Dynamic Rules
! △NOTE
Activate and Dynamic rules are being phased out in favor of a combination of tagging (3. \ content:"|E8C0FFFFFF|/bin".2.
3. but they have a different option ﬁeld: activated by. the direction operator did not have proper error checking and many people used an invalid token. This is handy for recording/analyzing both sides of a conversation. There is also a bidirectional operator.7: Activate/Dynamic Rule Example
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. This is very useful if you want to set Snort up to perform follow on recording when a speciﬁc rule goes off.does not exist is so that rules always read consistently.168. Activate rules act just like alert rules. Rule option keywords are separated from their arguments with a colon (:) character. there’s a very good possibility that useful data will be contained within the next 50 (or whatever) packets going to that same service port on the network.log tcp any any -> 192. Also.

com/bid/ http://cve.mitre.4 General Rule Options
3.nai.
3.”
3.) alert tcp any any -> any 21 (msg:"IDS287/ftp-wuftp260-venglin-linux".com/vil/content/v http://osvdb. \ flags:AP. reference:arachnids.securityfocus.org/plugins/dump.php3?id= (currently down) http://www.org/pub-bin/sigs-search. character).1: Supported Systems URL Preﬁx http://www.There are four major categories of rule options. \
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.4.org/show/osvdb/ http://
System bugtraq cve nessus arachnids mcafee osvdb url
Format reference:<id system>.snort. It is a simple text string that utilizes the \ as an escape character to indicate a discrete character that might otherwise confuse Snort’s rules parser (such as the semi-colon .cgi/ for a system that is indexing descriptions of alerts based on of the sid (See Section 3.1 msg
The msg rule option tells the logging and alerting engine the message to print along with a packet dump or to an alert.4.IDS411.2 reference
The reference keyword allows rules to include references to external attack identiﬁcation systems.whitehats. Make sure to also take a look at http://www. This plugin is to be used by output plugins to provide a link to additional information about the alert produced. Format msg:"<message text>".4. The plugin currently supports several speciﬁc systems as well as unique URLs. <id>.] Examples alert tcp any any -> any 7070 (msg:"IDS411/dos-realaudio".4).org/cgi-bin/cvename.com/info/IDS http://vil. general These options provide information about the rule but do not have any affect during detection payload These options all look for data inside the packet payload and can be inter-related non-payload These options look for non-payload data post-detection These options are rule speciﬁc triggers that happen after a rule has “ﬁred.nessus. <id>.cgi?name= http://cgi. [reference:<id system>. Table 3. content:"|fff4 fffd 06|".

This information is useful when postprocessing alert to map an ID to an alert message.999 Rules included with the Snort distribution • >=1. See etc/generators in the source tree for the current generator ids in use. it is recommended that values starting at 1.map contains a mapping of alert messages to Snort rule IDs. This option should be used with the sid keyword. it is not recommended that the gid keyword be used.000 be used.5) • <100 Reserved for future use • 100-999. gid:1000001.3 gid
The gid keyword (generator id) is used to identify what part of Snort generates the event when a particular rule ﬁres. This information allows output plugins to identify rules easily. Format gid:<generator id>.1387. alert tcp any any -> any 80 (content:"BOB".flags:AP.)
3.CAN-2000-1574. To avoid potential conﬂict with gids deﬁned in Snort (that for some reason aren’t noted it etc/generators). Note that the gid keyword is optional and if it is not speciﬁed in a rule. Example This example is a rule with the Snort Rule ID of 1000983. sid:1000983.000. content:"|31c031db 31c9b046 cd80 31c031db|".000. This option should be used with the rev keyword.4 sid
The sid keyword is used to uniquely identify Snort rules.)
3. alert tcp any any -> any 80 (content:"BOB".map contains contains more information on preprocessor and decoder gids. sid:1. it will default to 1 and the rule will be part of the general rule subsystem. \ reference:arachnids.)
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.IDS287. (See section 3. \ reference:cve. For example gid 1 is associated with the rules subsystem and various gids over 100 are designated for speciﬁc preprocessors and the decoder. rev:1.4) The ﬁle etc/gen-msg. Format sid:<snort rules id>.4. reference:bugtraq. (See section 3.000 Used for local rules The ﬁle sid-msg. rev:1.4.4.4. For general rule writing. Example This example is a rule with a generator id of 1000001.

) Attack classiﬁcations deﬁned by Snort reside in the classification. This option should be used with the sid keyword. \ content:"expn root". They are currently ordered with 4 default priorities.4.4) Format rev:<revision integer>. Revisions. Example alert tcp any any -> any 25 (msg:"SMTP expn root". Snort provides a default set of attack classes that are used by the default set of rules it provides.)
3. alert tcp any any -> any 80 (content:"BOB". Format classtype:<class name>. nocase.<class description>. rev:1. Table 3. flags:A+. A priority of 1 (high) is the most severe and 4 (very low) is the least severe. along with Snort rule id’s.6 classtype
The classtype keyword is used to categorize a rule as detecting an attack that is part of a more general type of attack class.4.config ﬁle.2.2: Snort Default Classiﬁcations Classtype attempted-admin attempted-user inappropriate-content policy-violation shellcode-detect successful-admin successful-user trojan-activity unsuccessful-user web-application-attack Description Attempted Administrator Privilege Gain Attempted User Privilege Gain Inappropriate Content was Detected Potential Corporate Privacy Violation Executable code was detected Successful Administrator Privilege Gain Successful User Privilege Gain A Network Trojan was detected Unsuccessful User Privilege Gain Web Application Attack 142 Priority high high high high high high high high high high
. sid:1000983.3. Example This example is a rule with the Snort Rule Revision of 1. allow signatures and descriptions to be reﬁned and replaced with updated information.<default priority>
These attack classiﬁcations are listed in Table 3.5 rev
The rev keyword is used to uniquely identify revisions of Snort rules.4. classtype:attempted-recon. Deﬁning classiﬁcations for rules provides a way to better organize the event data Snort produces. The ﬁle uses the following syntax: config classification: <class name>. (See section 3.

priority:10. Format priority:<priority integer>.attempted-dos attempted-recon bad-unknown default-login-attempt denial-of-service misc-attack non-standard-protocol rpc-portmap-decode successful-dos successful-recon-largescale successful-recon-limited suspicious-ﬁlename-detect suspicious-login system-call-detect unusual-client-port-connection web-application-activity icmp-event misc-activity network-scan not-suspicious protocol-command-decode string-detect unknown tcp-connection
Attempted Denial of Service Attempted Information Leak Potentially Bad Trafﬁc Attempt to login by a default username and password Detection of a Denial of Service Attack Misc Attack Detection of a non-standard protocol or event Decode of an RPC Query Denial of Service Large Scale Information Leak Information Leak A suspicious ﬁlename was detected An attempted login using a suspicious username was detected A system call was detected A client was using an unusual port Access to a potentially vulnerable web application Generic ICMP event Misc activity Detection of a Network Scan Not Suspicious Trafﬁc Generic Protocol Command Decode A suspicious string was detected Unknown Trafﬁc A TCP connection was detected
medium medium medium medium medium medium medium medium medium medium medium medium medium medium medium medium low low low low low low low very low
Warnings The classtype option can only use classiﬁcations that have been deﬁned in snort. 143
. Examples of each case are given below. \ content:"/cgi-bin/phf".7 priority
The priority tag assigns a severity level to rules.conf by using the config classification option. Examples alert tcp any any -> any 80 (msg:"WEB-MISC phf attempt".
3. \ dsize:>128. flags:A+.config that are used by the rules it provides.) alert tcp any any -> any 80 (msg:"EXPLOIT ntpdx overflow".4. priority:10 ). A classtype rule assigns a default priority (deﬁned by the config classification option) that may be overridden with a priority rule. classtype:attempted-admin. Snort provides a default set of classiﬁcations in classification.

The gid keyword (generator id) is used to identify what part of Snort generates the event when a particular rule ﬁres. soid 3|12345.4. the rule is applied to that packet. Examples alert tcp any any -> any 80 (msg:"Shared Library Rule Example". \ metadata:engine shared. \ metadata:service http. otherwise. typically in a key-value format.) alert tcp any any -> any 80 (msg:"Shared Library Rule Example".8 metadata
The metadata tag allows a rule writer to embed additional information about the rule. When the value exactly matches the service ID as speciﬁed in the table.3.4: General rule option keywords Keyword msg reference gid Description The msg keyword tells the logging and alerting engine the message to print with the packet dump or alert.9 General Rule Quick Reference
Table 3. metadata:key1 value1. metadata:soid 3|12345. the second a single metadata keyword. Format The examples below show an stub rule from a shared library rule. The ﬁrst uses multiple metadata keywords. Table 3. with keys separated by commas. Certain metadata keys and values have meaning to Snort and are listed in Table 3.4. key2 value2.
.3: Snort Metadata Keys Description Indicate a Shared Library Rule Shared Library Rule Generator and SID Target-Based Service Identiﬁer
Key engine soid service
Value Format ”shared” gid|sid ”http”
! △NOTE
The service Metadata Key is only meaningful when a Host Atttribute Table is provided. \ metadata:engine shared.7 for details on the Host Attribute Table. the rule is not applied (even if the ports speciﬁed in the rule match). metadata:key1 value1. The reference keyword allows rules to include references to external attack identiﬁcation systems.3.) alert tcp any any -> any 80 (msg:"HTTP Service Rule Example". Keys other than those listed in the table are effectively ignored by Snort and can be free-form. 144
. Multiple keys are separated by a comma. while keys and values are separated by a space. See Section 2.)
3. with a key and a value.

Bytecode represents binary data as hexadecimal numbers and is a good shorthand method for describing complex binary data. It allows the user to set rules that search for speciﬁc content in the packet payload and trigger response based on that data. the alert will be triggered on packets that do not contain this content. The classtype keyword is used to categorize a rule as detecting an attack that is part of a more general type of attack class.
3. The priority keyword assigns a severity level to rules. the test is successful and the remainder of the rule option tests are performed. the Boyer-Moore pattern match function is called and the (rather computationally expensive) test is performed against the packet contents.
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. If data exactly matching the argument data string is contained anywhere within the packet’s payload. Be aware that this test is case sensitive. The example below shows use of mixed text and binary data in a Snort rule.) alert tcp any any -> any 80 (content:!"GET". typically in a key-value format. This allows rules to be tailored for less false positives. within:50. and there are only 5 bytes of payload and there is no ”A” in those 5 bytes.5 Payload Detection Rule Options
3. Whenever a content option pattern match is performed. Note that multiple content rules can be speciﬁed in one rule.)
! △NOTE
A ! modiﬁer negates the results of the entire content search. the result will return a match. \ "
Format content:[!]"<content string>". it can contain mixed text and binary data. The option data for the content keyword is somewhat complex. For example. This is useful when writing rules that want to alert on packets that do not match a certain pattern
! △NOTE
Also note that the following characters must be escaped inside a content rule: . Examples alert tcp any any -> any 139 (content:"|5c 00|P|00|I|00|P|00|E|00 5c|". If the rule is preceded by a !. The binary data is generally enclosed within the pipe (|) character and represented as bytecode.5. use isdataat as a pre-cursor to the content.1 content
The content keyword is one of the more important features of Snort. if using content:!"A". The metadata keyword allows a rule writer to embed additional information about the rule. The rev keyword is used to uniquely identify revisions of Snort rules. If there must be 50 bytes for a valid match. modiﬁers included.sid rev classtype priority metadata
The sid keyword is used to uniquely identify Snort rules.

A depth of 5 would tell Snort to only look for the speciﬁed pattern within the ﬁrst 5 bytes of the payload. depth modiﬁes the previous ‘content’ keyword in the rule. You can not use depth with itself. You can not use offset with itself. depth:20. offset. This keyword allows values from -65535 to 65535. offset:4. there must be a content in the rule before offset is speciﬁed. or within (to modify the same content). The value can also be set to a string value referencing a variable extracted by the byte extract keyword in the same rule. rawbytes.)
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.5 offset
The offset keyword allows the rule writer to specify where to start searching for a pattern within a packet. As this keyword is a modiﬁer to the previous content keyword. alert tcp any any -> any 80 (content:"cgi-bin/phf". or within (to modify the same content). Format depth:[<number>|<var_name>]. there must be a content in the rule before depth is speciﬁed. The offset and depth keywords may be used together. Example The following example shows use of a combined content.Example This example tells the content pattern matcher to look at the raw trafﬁc. alert tcp any any -> any 21 (msg:"Telnet NOP". instead of the decoded trafﬁc provided by the Telnet decoder.)
3. offset modiﬁes the previous ’content’ keyword in the rule.5. The value can also be set to a string value referencing a variable extracted by the byte extract keyword in the same rule. The maximum allowed value for this keyword is 65535. The offset and depth keywords may be used together.4 depth
The depth keyword allows the rule writer to specify how far into a packet Snort should search for the speciﬁed pattern. The minimum allowed value is 1. distance. Format offset:[<number>|<var_name>].
3. An offset of 5 would tell Snort to start looking for the speciﬁed pattern after the ﬁrst 5 bytes of the payload. This keyword allows values greater than or equal to the pattern length being searched. and depth search rule. content:"|FF F1|". As the depth keyword is a modiﬁer to the previous content keyword.5. distance.

You can not use within with itself. distance:1.7 within
The within keyword is a content modiﬁer that makes sure that at most N bytes are between pattern matches using the content keyword ( See Section 3.{1}DEF/.5.6 distance
The distance keyword allows the rule writer to specify how far into a packet Snort should ignore before starting to search for the speciﬁed pattern relative to the end of the previous pattern match.1 ). or depth (to modify the same content). It’s designed to be used in conjunction with the distance (Section 3. Examples This rule constrains the search of EFG to not go past 10 bytes past the ABC match.)
3.3. offset. Example The rule below maps to a regular expression of /ABC. alert tcp any any -> any any (content:"ABC".6) rule option.5). offset. The distance and within keywords may be used together. or depth (to modify the same content). This can be thought of as exactly the same thing as offset (See Section 3. alert tcp any any -> any any (content:"ABC". within:10. You can not use distance with itself.)
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. except it is relative to the end of the last pattern match instead of the beginning of the packet. The value can also be set to a string value referencing a variable extracted by the byte extract keyword in the same rule.5. The distance and within keywords may be used together. This keyword allows values from -65535 to 65535. Format within:[<byte_count>|<var_name>].5. content:"EFG".5. The maximum allowed value for this keyword is 65535.5. content:"DEF". This keyword allows values greater than or equal to pattern length being searched. Format distance:[<byte_count>|<var_name>]. The value can also be set to a string value referencing a variable extracted by the byte extract keyword in the same rule.

The amount of data that is inspected with this option depends on the post depth conﬁg option of HttpInspect.)
! △NOTE
The http cookie modiﬁer is not allowed to be used with the rawbytes or fast pattern modiﬁers for the same content. content:"EFG". When enable cookie is not speciﬁed. per the conﬁguration of HttpInspect (see 2. http_client_body.2. http_cookie. Examples This rule constrains the search for the pattern ”EFG” to the raw body of an HTTP client request.)
! △NOTE
The http client body modiﬁer is not allowed to be used with the rawbytes modiﬁer for the same content. As this keyword is a modiﬁer to the previous content keyword. there must be a content in the rule before http cookie is speciﬁed. the cookie still ends up in HTTP header. alert tcp any any -> any 80 (content:"ABC". content:"EFG". This keyword is dependent on the enable cookie conﬁg option.2. If enable cookie is not speciﬁed. The Cookie Header ﬁeld will be extracted only when this option is conﬁgured. using http cookie is the same as using http header. Format http_cookie.5. there must be a content in the rule before ’http client body’ is speciﬁed.6). Pattern matches with this keyword wont work when post depth is set to -1.6).9 http cookie
The http cookie keyword is a content modiﬁer that restricts the search to the extracted Cookie Header ﬁeld of a HTTP client request or a HTTP server response (per the conﬁguration of HttpInspect 2.8 http client body
The http client body keyword is a content modiﬁer that restricts the search to the body of an HTTP client request. Format http_client_body. alert tcp any any -> any 80 (content:"ABC". The cookie buffer also includes the header name (Cookie for HTTP requests or Set-Cookie for HTTP responses).3. The extracted Cookie Header ﬁeld may be NORMALIZED.5.
3. Examples This rule constrains the search for the pattern ”EFG” to the extracted Cookie Header ﬁeld of a HTTP client request. As this keyword is a modiﬁer to the previous content keyword.
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.

2. Examples This rule constrains the search for the pattern ”EFG” to the extracted Header ﬁelds of a HTTP client request or a HTTP server response.
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.5.3. content:"EFG".)
! △NOTE
The http header modiﬁer is not allowed to be used with the rawbytes modiﬁer for the same content.6). there must be a content in the rule before http raw cookie is speciﬁed. As this keyword is a modiﬁer to the previous content keyword.
3.2.5. The extracted Header ﬁelds may be NORMALIZED.)
! △NOTE
The http raw cookie modiﬁer is not allowed to be used with the rawbytes. The Cookie Header ﬁeld will be extracted only when this option is conﬁgured.6). http_header.6).2. http_raw_cookie. Format http_raw_cookie.10 http raw cookie
The http raw cookie keyword is a content modiﬁer that restricts the search to the extracted UNNORMALIZED Cookie Header ﬁeld of a HTTP client request or a HTTP server response (per the conﬁguration of HttpInspect 2. content:"EFG". per the conﬁguration of HttpInspect (see 2. alert tcp any any -> any 80 (content:"ABC". This keyword is dependent on the enable cookie conﬁg option. As this keyword is a modiﬁer to the previous content keyword. Format http_header.11 http header
The http header keyword is a content modiﬁer that restricts the search to the extracted Header ﬁelds of a HTTP client request or a HTTP server response (per the conﬁguration of HttpInspect 2. there must be a content in the rule before http header is speciﬁed. Examples This rule constrains the search for the pattern ”EFG” to the extracted Unnormalized Cookie Header ﬁeld of a HTTP client request. http cookie or fast pattern modiﬁers for the same content. alert tcp any any -> any 80 (content:"ABC".

3. alert tcp any any -> any 80 (content:"ABC". The http method
3.2.5.3.12 http raw header
The http raw header keyword is a content modiﬁer that restricts the search to the extracted UNNORMALIZED Header ﬁelds of a HTTP client request or a HTTP server response (per the conﬁguration of HttpInspect 2. As this keyword is a modiﬁer to the previous content keyword. there must be a content in the rule before http method is speciﬁed. there must be a content in the rule before http uri is speciﬁed. http_method. As this keyword is a modiﬁer to the previous content keyword.5.)
! △NOTE modiﬁer is not allowed to be used with the rawbytes modiﬁer for the same content.13 http method
The http method keyword is a content modiﬁer that restricts the search to the extracted Method from a HTTP client request. Examples This rule constrains the search for the pattern ”EFG” to the extracted Header ﬁelds of a HTTP client request or a HTTP server response.20). http header or fast pattern The http raw
modiﬁers for the same content.5. there must be a content in the rule before http raw header is speciﬁed. http_raw_header. Format http_raw_header.5. Format http_method.14 http uri
The http uri keyword is a content modiﬁer that restricts the search to the NORMALIZED request URI ﬁeld . Using a content rule option followed by a http uri modiﬁer is the same as using a uricontent by itself (see: 3. alert tcp any any -> any 80 (content:"ABC".)
! △NOTE header modiﬁer is not allowed to be used with the rawbytes. content:"GET".6). content:"EFG". As this keyword is a modiﬁer to the previous content keyword. Examples This rule constrains the search for the pattern ”GET” to the extracted Method from a HTTP client request. 151
.

Examples This rule constrains the search for the pattern ”EFG” to the UNNORMALIZED URI. http_raw_uri.
3. As this keyword is a modiﬁer to the previous content keyword. http uri or fast pattern modiﬁers for the same content. there must be a content in the rule before http raw uri is speciﬁed.5.Format http_uri.
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.16 http stat code
The http stat code keyword is a content modiﬁer that restricts the search to the extracted Status code ﬁeld from a HTTP server response. alert tcp any any -> any 80 (content:"ABC". there must be a content in the rule before http stat code is speciﬁed. content:"EFG". Examples This rule constrains the search for the pattern ”EFG” to the NORMALIZED URI. content:"EFG". Format http_stat_code. As this keyword is a modiﬁer to the previous content keyword.15 http raw uri
The http raw uri keyword is a content modiﬁer that restricts the search to the UNNORMALIZED request URI ﬁeld .)
! △NOTE
The http uri modiﬁer is not allowed to be used with the rawbytes modiﬁer for the same content. http_uri.
3. The Status Code ﬁeld will be extracted only if the extended reponse inspection is conﬁgured for the HttpInspect (see 2.2. Format http_raw_uri.6).5.)
! △NOTE
The http raw uri modiﬁer is not allowed to be used with the rawbytes. alert tcp any any -> any 80 (content:"ABC".

Format http_stat_msg.6). alert tcp any any -> any 80 (content:"ABC".)
! △NOTE
The http stat code modiﬁer is not allowed to be used with the rawbytes or fast pattern modiﬁers for the same content. content:"Not Found". This rule option will not be able to detect encodings if the speciﬁed HTTP ﬁelds are not NORMALIZED. The keywords ’uri’.2. Negation is allowed on these keywords.
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. The Status Message ﬁeld will be extracted only if the extended reponse inspection is conﬁgured for the HttpInspect (see 2. ’header’ and ’cookie’ determine the HTTP ﬁelds used to search for a particular encoding type. Examples This rule constrains the search for the pattern ”Not Found” to the extracted Status Message ﬁeld of a HTTP server response. ’base36’. ’ascii’. These keywords can be combined using a OR operation. ’uencode’. http_stat_msg.2. There are eleven keywords associated with http encode. http_stat_code.6).)
! △NOTE
The http stat msg modiﬁer is not allowed to be used with the rawbytes or fast pattern modiﬁers for the same content.5. ’double encode’.
3. there must be a content in the rule before http stat msg is speciﬁed.17 http stat msg
The http stat msg keyword is a content modiﬁer that restricts the search to the extracted Status Message ﬁeld from a HTTP server response.18 http encode
The http encode keyword will enable alerting based on encoding type present in a HTTP client request or a HTTP server response (per the conﬁguration of HttpInspect 2. The conﬁg option ’normalize headers’ needs to be turned on for rules to work with the keyword ’header’.2.5. ’non ascii’. The keyword ’cookie’ is dependent on conﬁg options ’enable cookie’ and ’normalize cookies’ (see 2.Examples This rule constrains the search for the pattern ”200” to the extracted Status Code ﬁeld of a HTTP server response. The keywords ’utf8’.
3. alert tcp any any -> any 80 (content:"ABC". content:"200". ’iis encode’ and ’bare byte’ determine the encoding type which would trigger the alert.6). As this keyword is a modiﬁer to the previous content keyword.

The fast pattern matcher is used to select only those rules that have a chance of matching by using a content in the rule for selection and only evaluating that rule if the content is found in the payload.!utf8. http stat code. it can signiﬁcantly reduce the number of rules that need to be evaluated and thus increases performance. http_encode:uri. http_encode:uri. http raw cookie. that it is okay to use the fast pattern modiﬁer if another http content modiﬁer not mentioned above is used in combination with one of the above to modify the same content. http raw uri. The better the content used for the fast pattern matcher. Note. however. The OR and negation operations work only on the encoding type ﬁeld and not on http buffer type ﬁeld.
3. [!][<utf8|double_encode|non_ascii|base36|uencode|bare_byte|ascii|iis_e Examples alert tcp any any -> any any (msg:"UTF8/UEncode Encoding present". there must be a content rule option in the rule before fast pattern is speciﬁed. http stat msg.
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. Check for the speciﬁed encoding type in HTTP request or HTTP response header ﬁelds (depending on the packet ﬂow) Check for the speciﬁed encoding type in HTTP request or HTTP response cookie header ﬁelds (depending on the packet ﬂow) Check for utf8 encoding in the speciﬁed buffer Check for double encoding in the speciﬁed buffer Check for non-ASCII encoding in the speciﬁed buffer Check for base36 encoding in the speciﬁed buffer Check for u-encoding in the speciﬁed buffer Check for bare byte encoding in the speciﬁed buffer Check for ascii encoding in the speciﬁed buffer Check for IIS Unicode encoding in the speciﬁed buffer
http_encode:<http buffer type>.5. Since the default behavior of fast pattern determination is to use the longest content in the rule. The fast pattern option may be speciﬁed only once per rule. it is useful if a shorter content is more ”unique” than the longer content. Though this may seem to be overhead.Option uri header cookie utf8 double encode non ascii base36 uencode bare byte ascii iis encode Format
Description Check for the speciﬁed encoding type in HTTP client request URI ﬁeld.19 fast pattern
The fast pattern keyword is a content modiﬁer that sets the content within a rule to be used with the fast pattern matcher. http raw header. [!]<encoding type> http_encode:[uri|header|cookie]. meaning the shorter content is less likely to be found in a packet than the longer content.utf8|uencode.
! △NOTE
The fast pattern modiﬁer cannot be used with the following http content modiﬁers: http cookie. the less likely the rule will needlessly be evaluated. As this keyword is a modiﬁer to the previous content keyword.)
! △NOTE
Negation(!) and OR(|) operations cannot be used in conjunction with each other for the http encode keyword.) alert tcp any any -> any any (msg:"No UTF8".

The optional argument only can be used to specify that the content should only be used for the fast pattern matcher and should not be evaluated as a rule option. nocase. such as %2f or directory traversals.! △NOTE modiﬁer can be used with negated contents only if those contents are not modiﬁed with The fast pattern
offset.) This rule says to use the content ”IJKLMNO” for the fast pattern matcher and that the content should only be used for the fast pattern matcher and not evaluated as a content rule option. The reason is that the things you are looking for are normalized out of the URI buffer. fast_pattern. content:"IJKLMNO". the meaning is simply to use the speciﬁed content as the fast pattern content for the rule. fast_pattern:only.
! △NOTE arguments only and <offset>. even though it is shorter than the earlier pattern ”ABCDEFGH”.5. Note that (1) the modiﬁed content must be case insensitive since patterns are inserted into the pattern matcher in a case insensitive manner. depth.<length> are mutually exclusive. but still evaluate the content rule option as ”IJKLMNO”. content:"IJKLMNO". (2) negated contents cannot be used and (3) contents cannot have any positional modiﬁers such as offset.<length> can be used to specify that only a portion of the content should be used for the fast pattern matcher. fast_pattern. This is equivalent to using the http uri modiﬁer to a content keyword. these rules will not alert. for example.<length>. content:"IJKLMNO". fast_pattern:1. When used alone. This is useful. if a known content must be located in the payload independent of location in the payload. fast_pattern:only.)
3. depth. distance or within. alert tcp any any -> any 80 (content:"ABCDEFGH". The optional
Examples This rule causes the pattern ”IJKLMNO” to be used with the fast pattern matcher. alert tcp any any -> any 80 (content:"ABCDEFGH". as it saves the time necessary to evaluate the rule option. As such if you are writing rules that include things that are normalized. Format The fast pattern option can be used alone or optionally take arguments. distance or within.) This rule says to use ”JKLMN” as the fast pattern content. For example. alert tcp any any -> any 80 (content:"ABCDEFGH".20 uricontent
The uricontent keyword in the Snort rule language searches the NORMALIZED request URI ﬁeld. fast_pattern:<offset>. the URI: 155
. This is useful if the pattern is very long and only a portion of the pattern is necessary to satisfy ”uniqueness” thus reducing the memory required to store the entire pattern in the fast pattern matcher.5. The optional argument <offset>.

%252fp%68f? will get normalized into: /cgi-bin/phf? When writing a uricontent rule..4 offset 3.7 fast pattern 3.5. urilen:[<|>]<number>.21 urilen
The urilen keyword in the Snort rule language speciﬁes the exact length..5 distance 3. If you wish to uricontent
search the UNNORMALIZED request URI ﬁeld.6. The following example will match URIs that are 5 bytes long: 156
.exe?/c+ver will get normalized into: /winnt/system32/cmd. (See Section 3. Format uricontent:[!]"<content string>". write the content that you want to ﬁnd in the context that the URI will be normalized.
3.6: Uricontent Modiﬁers Modiﬁer Section nocase 3.%c0%af.1) uricontent can be used with several of the modiﬁers available to the content keyword.
! △NOTE cannot be modiﬁed by a rawbytes modiﬁer or any of the other HTTP modiﬁers. These include: Table 3. the minimum length. For example. if Snort normalizes directory traversals. use the http raw uri modiﬁer with a content option. do not include directory traversals. Format urilen:min<>max./scripts/.5.exe?/c+ver Another example.5.2 depth 3.5.5.6 within 3.5.5./winnt/system32/cmd.19
This option works in conjunction with the HTTP Inspect preprocessor speciﬁed in Section 2. the maximum length.. You can write rules that look for the non-normalized content by using the content option. the URI: /cgi-bin/aaaaaaaaaaaaaaaaaaaaaaaaaa/.2. or range of URI lengths to match.5.

relative|rawbytes]. The following example will match URIs that are shorter than 5 bytes: urilen:<5. S and Y. then veriﬁes there is at least 50 bytes after the end of the string PASS. This modiﬁer will work with the relative modiﬁer as long as the previous content match was in the raw packet data. K. I. the rule with modiﬁers content:"foo".9 for descriptions of each modiﬁer.7. would alert if there were not 10 bytes after ”foo” before the payload ended. D.
157
. The following example will match URIs that are greater than 5 bytes and less than 10 bytes: urilen:5<>10. check out the PCRE web site http://www.22 isdataat
Verify that the payload has data at a speciﬁed location. Format isdataat:[!]<int>[.
3.urilen:5.) This rule looks for the string PASS exists in the packet. P. For more detail on what can be done via a pcre regular expression. \ content:!"|0a|". The modiﬁers
H. optionally looking for data relative to the end of the previous content match.org Format pcre:[!]"(/<regex>/|m<delim><regex><delim>)[ismxAEGRUBPHMCOIDKYS]". C. The post-re modiﬁers set compile time ﬂags for the regular expression. 3. This option works in conjunction with the HTTP Inspect preprocessor speciﬁed in Section 2.2.relative. then veriﬁes that there is not a newline character within 50 bytes of the end of the PASS string. within:50. When the rawbytes modiﬁer is speciﬁed with isdataat.pcre.relative. For example. Example alert tcp any any -> any 111 (content:"PASS". A ! modiﬁer negates the results of the isdataat test. ignoring any decoding that was done by the preprocessors.8.5. isdataat:!10. and 3.5. it looks at the raw packet data.6. M.23 pcre
The pcre keyword allows rules to be written using perl compatible regular expressions. See tables 3.
3. It will alert if a certain amount of data is not present within the payload. isdataat:50.
! △NOTE R (relative) and B (rawbytes) are not allowed with any of the HTTP modiﬁers such as U.

This is dependent on the SMTP conﬁg option enable mime decoding.)
! △NOTE
Snort’s handling of multiple URIs with PCRE does not work as expected.2. as well as the very start and very end of the buffer. In order to use pcre to inspect all URIs. whitespace data characters in the pattern are ignored except when escaped or inside a character class
A E
G
Table 3. Without E.
3. This option will operate similarly to the dce stub data option added with DCE/RPC2. ˆ and $ match at the beginning and ending of the string. byte jump.6 for more details. $ also matches immediately before the ﬁnal character if it is a newline (but not before any other newlines). pcre) to use. the string is treated as one big line of characters. This option matches if there is HTTP response body or SMTP body or SMTP MIME base64 decoded data.7: Perl compatible modiﬁers for pcre case insensitive include newlines in the dot metacharacter By default. alert ip any any -> any any (pcre:"/BLAH/i". When m is set. you must use either a content or a uricontent.7 for more details.2.24 ﬁle data
This option is used to place the cursor (used to walk the packet payload in rules processing) at the beginning of either the entity body of a HTTP response or the SMTP body data. ˆ and $ match immediately following or immediately before any newline in the buffer. Inverts the ”greediness” of the quantiﬁers so that they are not greedy by default. file_data:mime.
158
. See 2. PCRE when used without a uricontent only evaluates the ﬁrst URI. When used with argument mime it places the cursor at the beginning of the base64 decoded MIME attachment or base64 decoded MIME body. Format file_data. certain HTTP Inspect options such as extended response inspection and inspect gzip (for decompressed gzip data) needs to be turned on. in that it simply sets a reference for other relative rule options ( byte test.5.
Example This example performs a case-insensitive search for the string BLAH in the payload.i s m
x
Table 3. For this option to work with HTTP response. See 2.8: PCRE compatible modiﬁers for pcre the pattern must match only at the start of the buffer (same as ˆ ) Set $ to match only at the end of the subject string. This file data can point to either a ﬁle or a block of data. but become greedy if followed by ”?”.

Match the unnormalized HTTP request uri buffer (Similar to http raw uri). within:10.9: Snort speciﬁc modiﬁers for pcre Match relative to the end of the last pattern match.) alert tcp any any -> any any(msg:"MIME BASE64 Encoded Data".
159
. Match normalized HTTP request method (Similar to http method) Match normalized HTTP request or HTTP response cookie (Similar to http cookie).25 base64 decode
This option is used to decode the base64 encoded data. ][offset <offset>[. Format base64_decode[:[bytes <bytes_to_decode>][.
! △NOTE
Multiple base64 encoded attachments in one packet are pipelined. This modiﬁer is not allowed with the normalized HTTP request or HTTP response header modiﬁer(H) for the same content. This option unfolds the data before decoding it.3). This modiﬁer is not allowed with the unnormalized HTTP request or HTTP response header modiﬁer(D) for the same content.
Example alert tcp any 80 -> any any(msg:"foo at the start of http response body". relative]]]. This modiﬁer is not allowed with the unnormalized HTTP request uri buffer modiﬁer(I) for the same content. within:3. This option is particularly useful in case of HTTP headers such as HTTP authorization headers.1. (Similar to distance:0. It completely ignores the limits while evaluating the pcre pattern speciﬁed. nocase.) Match the decoded URI buffers (Similar to uricontent and http uri). content:"foo".R U
I
P H
D
M C
K
S Y B O
Table 3. Match unnormalized HTTP request body (Similar to http client body) Match normalized HTTP request or HTTP response header (Similar to http header).\ file_data:mime. Match HTTP response status code (Similar to http stat code) Match HTTP response status message (Similar to http stat msg) Do not use the decoded buffers (Similar to rawbytes) Override the conﬁgured pcre match limit and pcre match limit recursion for this expression (See section 2. \ file_data.5. Match unnormalized HTTP request or HTTP response cookie (Similar to http raw cookie). content:"foo". This modiﬁer is not allowed with the HTTP request uri buffer modiﬁer(U) for the same content. Match unnormalized HTTP request or HTTP response header (Similar to http raw header). This modiﬁer is not allowed with the normalized HTTP request or HTTP response cookie modiﬁer(C) for the same content.)
3. This modiﬁer is not allowed with the unnormalized HTTP request or HTTP response cookie modiﬁer(K) for the same content.

) alert tcp $EXTERNAL_NET any -> $HOME_NET any \ (msg:"Authorization NTLM". This option matches if there is base64 decoded buffer. byte jump.)
3.26 base64 data
This option is used to place the cursor (used to walk the packet payload in rules processing) at the beginning of the base64 decode buffer if present. base64_data. content:"NTLMSSP". offset 6. http_header.Option bytes
offset
relative
Description Number of base64 encoded bytes to decode.
The above arguments to base64 decode are optional. content:"Authorization: NTLM". When this option is not speciﬁed we look for base64 encoded data till either the end of header line is reached or end of packet payload is reached. Fast pattern content matches are not allowed with this buffer. This option needs to be used in conjunction with base64 data for any other relative rule options to work on base64 decoded buffer. This argument takes positive and non-zero values only. in that it simply sets a reference for other relative rule options ( byte test. If folding is not present the search for base64 encoded data will end when we see a carriage return or line feed or both without a following space or tab.
Examples alert tcp $EXTERNAL_NET any -> $HOME_NET any \ (msg:"Base64 Encoded Data".) alert tcp any any -> any any (msg:"Authorization NTLM". base64_decode:relative. \ content:"NTLMSSP". Speciﬁes the inspection for base64 encoded data is relative to the doe ptr. The rule option base64 decode needs to be speciﬁed before the base64 data option. within:8. Determines the offset relative to the doe ptr when the option relative is speciﬁed or relative to the start of the packet payload to begin inspection of base64 encoded data. within:20. Format base64_data. \ content:"Authorization:".
! △NOTE
Any non-relative rule options in the rule will reset the cursor(doe ptr) from base64 decode buffer.
160
. This option does not take any arguments. pcre) to use. base64_decode. This option will operate similarly to the file data option.
! △NOTE
This option can be extended to protocols with folding similar to HTTP. \ base64_decode:bytes 12. This argument takes positive and non-zero values only. \ content:"foo bar". relative. base64_data.5. base64_data. \ within:20.

Format
byte_test:<bytes to convert>.}
Examples
alert udp $EXTERNAL_NET any -> $HOME_NET any \
161
.4294967295 -65535 to 65535 Description Number of bytes to pick up from the packet. \ content:"NTLMSSP". bytes operator value offset = = = = 1 . \ base64_decode:bytes 12.bitwise AND • ˆ .Process data as little endian string number type Data is stored in string format in packet Type of number being read: • hex . relative][. If used with dce allowed values are 1. <number type>][.
Option bytes to convert operator
Any of the operators can also include ! to check if the operator is not true. then it would be the same as using if (data & value) { do something().13 for a description and examples (2. Capable of testing binary values or converting representative byte strings to their binary equivalent and testing them.Converted string data is represented in octal dce Let the DCE/RPC 2 preprocessor determine the byte order of the value to be converted. http_header. please read Section 3.10 ’<’ | ’=’ | ’>’ | ’&’ | ’ˆ’ 0 . then the operator is set to =. relative. <offset> \ [.greater than • = . For a more detailed explanation.Process data as big endian (default) • little .)
3. \ content:"Authorization:".Example alert tcp any any -> any any (msg:"Authorization NTLM".Converted string data is represented in decimal • oct . The allowed values are 1 to 10 when used without dce.equal • & .Converted string data is represented in hexadecimal • dec .bitwise OR value offset relative endian Value to test the converted value against Number of bytes into the payload to start processing Use an offset relative to last pattern match Endian type of the number being read: • big .2.less than • > . Operation to perform to test the value: • < . string. <endian>][. base64_data. <value>. If ! is speciﬁed without an operator.5.
! △NOTE
Snort uses the C operators for each of these operators. If the & operator is used. dce].9. offset 6.13 for quick reference). 2 and 4. [!]<operator>. within:8. See section 2.27 byte test
Test a byte ﬁeld against a speciﬁc value (with operator).5.2.

)
3.5. >. <number_type>][. Let the DCE/RPC 2 preprocessor determine the byte order of the value to be converted. \ content:"|00 00 00 01|".13 for quick reference). Here is a list of places where byte extract variables can be used:
163
. This will be used to reference the variable in other rule options. Use an offset relative to last pattern match Multiply the bytes read from the packet by <value> and save that number into the variable. The allowed values are 1 to 10 when used without dce.
Example
alert udp any any -> any 32770:34000 (content:"|00 01 86 B8|". The DCE/RPC 2 preprocessor must be enabled for this option to work. 2 and 4. \ msg:"statd format string buffer overflow".
Other options which use byte extract variables
A byte extract rule option detects nothing by itself. \ byte_test:4. dce] Option bytes to convert offset name relative multiplier <value> big little dce string hex dec oct align <value> Description Number of bytes to pick up from the packet Number of bytes into the payload to start processing Name of the variable. Process data as big endian (default) Process data as little endian Use the DCE/RPC 2 preprocessor to determine the byte-ordering. 20. <value> may be 2 or 4. relative. \ byte_jump:4. align <align value>][. Process data as big endian (default) Process data as little endian Data is stored in string format in packet Converted string data is represented in hexadecimal Converted string data is represented in decimal Converted string data is represented in octal Round the number of converted bytes up to the next 32-bit boundary Skip forward from the beginning of the packet payload instead of from the current position in the packet. <name> \ [. relative][.2. instead of using hard-coded values. multiplier <multiplier value>][. distance:4. Its use is in extracting packet data for use in other rule options. Skip forward or backwards (positive of negative value) by <value> number of bytes after the other jump options have been applied. See section 2.13 for a description and examples (2. They can be re-used in the same rule any number of times. 12. string. <offset>. These variables can be referenced later in the rule. relative. Data is stored in string format in packet Converted string data is represented in hexadecimal Converted string data is represented in decimal Converted string data is represented in octal Round the number of converted bytes up to the next <value>-byte boundary. <endian>]\ [.
! △NOTE
Only two byte extract variables may be created per rule.
Format
byte_extract:<bytes_to_extract>. It reads in some number of bytes from the packet payload and saves it to a variable. within:4. Number of bytes into the payload to start processing Use an offset relative to last pattern match Multiply the number of calculated bytes by <value> and skip forward that number of bytes.Option bytes to convert offset relative multiplier <value> big little string hex dec oct align from beginning post offset <value> dce
Description Number of bytes to pick up from the packet.2. align. 900.29 byte extract
The byte extract keyword is another useful option for writing rules against length-encoded protocols. If used with dce allowed values are 1.

• Use these values to constrain a pattern match to a smaller area.
Format
ftpbounce. if you wanted to decode snmp packets. So if you wanted to start decoding and ASN. \ msg:"Bad Stuff detected within field". alert tcp any any -> any any (byte_extract:1.Rule Option content/uricontent byte test byte jump isdataat
Arguments that Take Variables offset. str_depth. The syntax looks like. oversize_length <value>][.
Example
alert tcp $EXTERNAL_NET any -> $HOME_NET 21 (msg:"FTP PORT bounce attempt". but it is unknown at this time which services may be exploitable.\ classtype:misc-attack. absolute_offset <value>|relative_offset <value>]. \ flow:to_server. absolute offset has one argument. \ byte_extract:1.
absolute offset <value>
relative offset <value>
164
.)
3. the option and the argument are separated by a space or a comma.5.1 detection plugin decodes a packet or a portion of a packet. \ content:"bad stuff". relative offset has one argument. content:"PORT". Detects a double ASCII encoding that is larger than a standard buffer. the offset number. This means that if an ASN. 1. then this keyword is evaluated as true. asn1:bitstring_overflow. depth:str_depth.31 asn1
The ASN. within offset.1 options provide programmatic detection capabilities as well as some more dynamic type detection. value offset offset
Examples
This example uses two variables to: • Read the offset of a string from a byte at offset 0.)
3. str_offset.established. This is the absolute offset from the beginning of the packet.1 sequence right after the content “foo”.30 ftpbounce
The ftpbounce keyword detects FTP bounce attacks. This is known to be an exploitable function in Microsoft. This is the relative offset from the last content match or byte test/jump. • Read the depth of a string from a byte at offset 1. The preferred usage is to use a space between option and argument.5. This keyword must have one argument which speciﬁes the length to compare against. For example. distance. Offset may be positive or negative. “oversize length 500”. nocase. relative_offset 0’.
Format
asn1:[bitstring_overflow][. double_overflow][. rev:1.1 type is greater than 500. Compares ASN. Option bitstring overflow double overflow oversize length <value> Description Detects invalid bitstring encodings that are known to be remotely exploitable. depth. pcre:"/ˆPORT/smi". The ASN. ftpbounce. and looks for various malicious encodings. So if any of the arguments evaluate as true. you would specify ’content:"foo". you would say “absolute offset 0”. the whole option evaluates as true. 0. Offset values may be positive or negative. If an option has an argument.1 type lengths with the supplied argument. Multiple options can be used in an ’asn1’ option and the implied logic is boolean OR. offset:str_offset. the offset value. sid:3441.

)
3.33 dce iface
See the DCE/RPC 2 Preprocessor section 2.5.Examples
alert udp any any -> any 161 (msg:"Oversize SNMP Length".13 for a description and examples of using this rule option.g. \ flow:to_server. relative_offset 0.34 dce opnum
See the DCE/RPC 2 Preprocessor section 2. \ asn1:oversize_length 10000.2.13 for a description and examples of using this rule option.5. Default CVS server ports are 2401 and 514 and are included in the default ports for stream reassembly.)
3.
Examples
alert tcp any any -> any 2401 (msg:"CVS Invalid-entry".11 for a description and examples of using this rule option.
3.5. \ asn1:bitstring_overflow.
! △NOTE
This plugin cannot do detection over encrypted sessions. SSH (usually port 22).13 for a description and examples of using this rule option.38 Payload Detection Quick Reference
Table 3.5.2.35 dce stub data
See the DCE/RPC 2 Preprocessor section 2.5. which is a way of causing a heap overﬂow (see CVE-2004-0396) and bad pointer derefenece in versions of CVS 1.15 and before.32 cvs
The CVS detection plugin aids in the detection of: Bugtraq-10384. Option invalid-entry Description Looks for an invalid Entry string.5.2. absolute_offset 0.2.11 for a description and examples of using this rule option.established.
3.
3.36 ssl version
See the SSL/TLS Preprocessor section 2. e.11. cvs:invalid-entry.
3. CVE-2004-0396: ”Malformed Entry Modiﬁed and Unchanged ﬂag insertion”.2.37 ssl state
See the SSL/TLS Preprocessor section 2.
Format
cvs:<option>.5.
165
.) alert tcp any any -> any 80 (msg:"ASN1 Relative Foo".
3. content:"foo".10: Payload detection rule option keywords Keyword content Description The content keyword allows the user to set rules that search for speciﬁc content in the packet payload and trigger response based on that data.

ignoring any decoding that was done by preprocessors. ttl:[<number>]-[<number>]. The byte test keyword tests a byte ﬁeld against a speciﬁc value (with operator).
Example
alert ip any any -> any any \ (msg:"First Fragment". The isdataat keyword veriﬁes that the payload has data at a speciﬁed location.2. The distance keyword allows the rule writer to specify how far into a packet Snort should ignore before starting to search for the speciﬁed pattern relative to the end of the previous pattern match. The cvs keyword detects invalid entry strings.6. =.
166
.1 fragoffset
The fragoffset keyword allows one to compare the IP fragment offset ﬁeld against a decimal value.)
3. The ftpbounce keyword detects FTP bounce attacks. To catch all the ﬁrst fragments of an IP session. See the DCE/RPC 2 Preprocessor section 2. you could use the fragbits keyword and look for the More fragments option in conjunction with a fragoffset of 0.
Format
fragoffset:[!|<|>]<number>.13.13. This keyword takes numbers from 0 to 255.
Format
ttl:[<.13. See the DCE/RPC 2 Preprocessor section 2. The depth keyword allows the rule writer to specify how far into a packet Snort should search for the speciﬁed pattern. This example checks for a time-to-live value that between 3 and 5.2 ttl
The ttl keyword is used to check the IP time-to-live value.2. The asn1 detection plugin decodes a packet or a portion of a packet.6. The within keyword is a content modiﬁer that makes sure that at most N bytes are between pattern matches using the content keyword. The byte jump keyword allows rules to read the length of a portion of data. ttl:<3. This option keyword was intended for use in the detection of traceroute attempts. fragoffset:0. The offset keyword allows the rule writer to specify where to start searching for a pattern within a packet. and looks for various malicious encodings. >=]<number>. See the DCE/RPC 2 Preprocessor section 2. The uricontent keyword in the Snort rule language searches the normalized request URI ﬁeld.6 Non-Payload Detection Rule Options
3. then skip that far forward in the packet.rawbytes depth offset distance within uricontent isdataat pcre byte test byte jump ftpbounce asn1 cvs dce iface dce opnum dce stub data
The rawbytes keyword allows rules to look at the raw packet data.
3. <=. >. The pcre keyword allows rules to be written using perl compatible regular expressions.
Example
This example checks for a time-to-live value that is less than 3. fragbits:M.2.

Format
id:<number>. The following examples are NOT allowed by ttl keyword: ttl:=>5.ttl:3-5.3 tos
The tos keyword is used to check the IP TOS ﬁeld for a speciﬁc value.
3. the value 31337 is very popular with some hackers.
167
.
3. ttl:>=5. Some tools (exploits. ttl:-5.
Example
This example looks for the IP ID of 31337.6.6. scanners and other odd programs) set this ﬁeld speciﬁcally for various purposes.
Example
This example looks for a tos value that is not 4 tos:!4. ttl:5-. for example. ttl:=5.4 id
The id keyword is used to check the IP ID ﬁeld for a speciﬁc value. ttl:5-3. ttl:=<5. This example checks for a time-to-live value that between 5 and 255. id:31337. This example checks for a time-to-live value that between 0 and 5.
Format
tos:[!]<number>. Few other examples are as follows: ttl:<=5.

More Fragments D .6.No Op ts .Record Route eol .Stream identiﬁer any .Don’t Fragment R .6 fragbits
The fragbits keyword is used to check if fragmentation and reserved bits are set in the IP header.
3.End of list nop .Strict Source Routing satid . The following bits may be checked: M .5 ipopts
The ipopts keyword is used to check if a speciﬁc IP option is present.any IP options are set The most frequently watched for IP options are strict and loose source routing which aren’t used in any widespread internet applications. The following options may be checked: rr .Reserved Bit The following modiﬁers can be set to change the match criteria: + match on the speciﬁed bits. ipopts:lsrr.
Example
This example looks for the IP Option of Loose Source Routing.
Format
ipopts:<rr|eol|nop|ts|sec|esec|lsrr|lsrre|ssrr|satid|any>. plus any others * match if any of the speciﬁed bits are set ! match if the speciﬁed bits are not set
Format
fragbits:[+*!]<[MDR]>.
Example
This example checks if the More Fragments bit and the Do not Fragment bit are set.
168
.Time Stamp sec .IP Extended Security lsrr .Loose Source Routing lsrre .6. fragbits:MD+.Loose Source Routing (For MS99-038 and CVE-1999-0909) ssrr .IP Security esec .
Warning
Only a single ipopts keyword may be speciﬁed per rule.3.

CWR .8 ﬂags
The ﬂags keyword is used to check if speciﬁc TCP ﬂag bits are present.match if any of the speciﬁed bits are set ! .7 dsize
The dsize keyword is used to test the packet payload size.
Warning
dsize will fail on stream rebuilt packets. CE ﬂag in IP header is set) 0 .
3.6. it is useful for detecting buffer overﬂows. alert tcp any any -> any any (flags:SF. dsize:300<>400.No TCP Flags Set The following modiﬁers can be set to change the match criteria: + .URG . The following bits may be checked: F . then ECN capable.6.Congestion Window Reduced (MSB in TCP Flags byte) 2 .Reset P . ignoring reserved bit 1 and reserved bit 2.12.Finish (LSB in TCP Flags byte) S .FIN .Urgent 1 .
Example
This example checks if just the SYN and the FIN bits are set.ACK .match on the speciﬁed bits.Synchronize sequence numbers R . regardless of the values of the reserved bits. Else.match if the speciﬁed bits are not set To handle writing rules for session initiation packets such as ECN where a SYN packet is sent with the previously reserved bits 1 and 2 set.ECE .SYN .RST .Push A . an option mask may be speciﬁed. A rule could check for a ﬂags value of S.ECN-Echo (If SYN.)
169
.
Format
dsize:min<>max.PSH . dsize:[<|>]<number>.
Format
flags:[!|*|+]<FSRPAU120>[. This may be used to check for abnormally sized packets.<FSRPAU12>].3.12 if one wishes to ﬁnd packets with just the syn bit.Acknowledgment U .
Example
This example looks for a dsize that is between 300 and 400 bytes. regardless of the size of the payload. In many cases. plus any others * .

regardless of the rest of the detection options. All the ﬂowbits in a particular group (with an exception of default group) are mutually exclusive. Most of the options need a user-deﬁned name for the speciﬁc state that is being checked. and underscores.6.2.
Examples
alert tcp !$HOME_NET any -> $HOME_NET 21 (msg:"cd incoming detected". This string should be limited to any alphanumeric string including periods. It allows rules to track states during a transport protocol session. as it allows rules to generically track the state of an application protocol. dashes. Checks if the speciﬁed state is set. There are eight keywords associated with ﬂowbits.(no_stream|only_stream)] [. When no group name is speciﬁed the ﬂowbits will belong to a default group. Option set unset toggle isset isnotset noalert reset Description Sets the speciﬁed state for the current ﬂow and unsets all the other ﬂowbits in a group when a GROUP NAME is speciﬁed.2). Unsets the speciﬁed state for the current ﬂow. content:"CWD incoming". \ flow:from_client.) alert tcp !$HOME_NET 0 -> $HOME_NET 0 (msg:"Port 0 TCP traffic".9 ﬂow
The ﬂow keyword is used in conjunction with TCP stream reassembly (see Section 2. \ flow:stateless. The established keyword will replace the flags:+A used in many places to show established TCP connections. Reset all states on a given ﬂow. The ﬂowbits option is most useful for TCP sessions. otherwise unsets the state if the state is set.
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. Checks if the speciﬁed state is not set. A particular ﬂow cannot belong to more than one group.3.2). The keywords set and toggle take an optional argument which speciﬁes the group to which the keywords will belong. Sets the speciﬁed state if the state is unset and unsets all the other ﬂowbits in a group when a GROUP NAME is speciﬁed. This allows packets related to $HOME NET clients viewing web pages to be distinguished from servers running in the $HOME NET.6. This allows rules to only apply to clients or servers.)
3.
Options
Option to client to server from client from server established not established stateless no stream only stream no frag only frag Description Trigger on server responses from A to B Trigger on client requests from A to B Trigger on client requests from A to B Trigger on server responses from A to B Trigger only on established TCP connections Trigger only when no TCP connection is established Trigger regardless of the state of the stream processor (useful for packets that are designed to cause machines to crash) Do not trigger on rebuilt stream packets (useful for dsize and stream5) Only trigger on rebuilt stream packets Do not trigger on rebuilt frag packets Only trigger on rebuilt frag packets
Format
flow:[(established|not_established|stateless)] [.10 ﬂowbits
The flowbits keyword is used in conjunction with conversation tracking from the Stream preprocessor (see Section2. It allows rules to only apply to certain directions of the trafﬁc ﬂow.(no_frag|only_frag)].(to_client|to_server|from_client|from_server)] [. Cause the rule to not generate an alert. nocase.2.

13 window
The window keyword is used to check for a speciﬁc TCP window size.
Format
ack:<number>.12 ack
The ack keyword is used to check for a speciﬁc TCP acknowledge number.) alert tcp any any -> any 143 (msg:"IMAP LIST". flowbits:noalert. <STATE_NAME>][.
Format
seq:<number>. content:"OK LOGIN".6.6.
Examples
alert tcp any 143 -> any any (msg:"IMAP login".
3.logged_in. <GROUP_NAME>].
3. content:"LIST".logged_in.
Example
This example looks for a TCP sequence number of 0.
Example
This example looks for a TCP window size of 55808.
Format
window:[!]<number>.
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. window:55808. seq:0. flowbits:set.Format
flowbits:[set|unset|toggle|isset|isnotset|noalert|reset][.)
3. ack:0.
Example
This example looks for a TCP acknowledge number of 0.11 seq
The seq keyword is used to check for a speciﬁc TCP sequence number.6. flowbits:isset.

17 icmp seq
The icmp seq keyword is used to check for a speciﬁc ICMP sequence value.15 icode
The icode keyword is used to check for a speciﬁc ICMP code value. icode:>30.6.3.
3. itype:>30.6. This particular plugin was developed to detect the stacheldraht DDoS agent.16 icmp id
The icmp id keyword is used to check for a speciﬁc ICMP ID value.
172
. itype:[<|>]<number>.
Example
This example looks for an ICMP code greater than 30.
Example
This example looks for an ICMP ID of 0.6. This is useful because some covert channel programs use static ICMP ﬁelds when they communicate.
3.
Example
This example looks for an ICMP type greater than 30.14 itype
The itype keyword is used to check for a speciﬁc ICMP type value.
Format
icode:min<>max.
Format
icmp_seq:<number>. This particular plugin was developed to detect the stacheldraht DDoS agent.6.
Format
icmp_id:<number>.
Format
itype:min<>max. This is useful because some covert channel programs use static ICMP ﬁelds when they communicate. icode:[<|>]<number>. icmp_id:0.
3.

Format
sameip.
Example
The following example looks for an RPC portmap GETPORT request.). [<version number>|*]. see /etc/protocols.19 ip proto
The ip proto keyword allows checks against the IP protocol header.
Format
ip_proto:[!|>|<] <name or number>. *.
3.
Format
rpc:<application number>.20 sameip
The sameip keyword allows rules to check if the source ip is the same as the destination IP.6.18 rpc
The rpc keyword is used to check for a RPC application. [<procedure number>|*]>.)
3. icmp_seq:0.
Example
This example looks for any trafﬁc where the Source IP and the Destination IP is the same. version. and procedure numbers in SUNRPC CALL requests. alert tcp any any -> any 111 (rpc:100000.6. the RPC keyword is slower than looking for the RPC values by using normal content matching.)
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.
Example
This example looks for IGMP trafﬁc. alert ip any any -> any any (sameip.6.
3.Example
This example looks for an ICMP Sequence of 0.
Warning
Because of the fast pattern matching engine. 3. Wildcards are valid for both version and procedure numbers by using ’*’. For a list of protocols that may be speciﬁed by name. alert ip any any -> any any (ip_proto:igmp.

11: Non-payload detection rule option keywords Keyword fragoffset ttl tos Description The fragoffset keyword allows one to compare the IP fragment offset ﬁeld against a decimal value.less than or equal • >= .noalert. The tos keyword is used to check the IP TOS ﬁeld for a speciﬁc value.6. to disable TCP reassembly for client trafﬁc when we see a HTTP 200 Ok Response message.<.not equal • <= .client. • The optional noalert parameter causes the rule to not generate an alert when it matches.greater than • = . Where the operator is one of the following: • < . use: alert tcp any 80 -> any any (flow:to_client.6. The ttl keyword is used to check the IP time-to-live value. fastpath].)
3.greater than or equal
Example
For example.)
3. <number>. established.3. noalert][.6.23 Non-Payload Detection Quick Reference
Table 3.
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.22 stream size
The stream size keyword allows a rule to match trafﬁc according to the number of bytes observed. use: alert tcp any any -> any any (stream_size:client. content:"200 OK".less than • > .
Format
stream_reassemble:<enable|disable>.6. <operator>. <server|client|both>[.
Example
For example.21 stream reassemble
The stream reassemble keyword allows a rule to enable or disable TCP stream reassembly on matching trafﬁc. to look for a session that is less that 6 bytes from the client side. stream_reassemble:disable.
! △NOTE
The stream size option is only available when the Stream5 preprocessor is enabled.
! △NOTE
The stream reassemble option is only available when the Stream5 preprocessor is enabled. • The optional fastpath parameter causes Snort to ignore the rest of the connection.
Format
stream_size:<server|client|both|either>. as determined by the TCP sequence numbers.equal • != .

or even web sessions is very useful.7. The ﬂags keyword is used to check if speciﬁc TCP ﬂag bits are present.7. There are many cases where seeing what users are typing in telnet. The dsize keyword is used to test the packet payload size. session:binary.
Example
The following example logs all printable strings in a telnet packet. The rpc keyword is used to check for a RPC application. The itype keyword is used to check for a speciﬁc ICMP type value. The icode keyword is used to check for a speciﬁc ICMP code value. this example logs the payload bytes in binary form. The fragbits keyword is used to check if fragmentation and reserved bits are set in the IP header.) Given an FTP data session on port 12345. There are three available argument keywords for the session rule option: printable. etc.)
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. The printable keyword only prints out data that the user would normally see or be able to type. and procedure numbers in SUNRPC CALL requests. log tcp any any <> any 23 (session:printable. log tcp any any <> any 12345 (metadata:service ftp-data. It should be noted that this option does not work when Snort is in binary logging mode.7 Post-Detection Rule Options
3. binary. HTTP CGI scans. The ack keyword is used to check for a speciﬁc TCP acknowledge number. The sameip keyword allows rules to check if the source ip is the same as the destination IP. The icmp id keyword is used to check for a speciﬁc ICMP ID value. The ip proto keyword allows checks against the IP protocol header. The ﬂowbits keyword allows rules to track states during a transport protocol session. This is especially handy for combining data from things like NMAP activity. The ipopts keyword is used to check if a speciﬁc IP option is present.
Format
logto:"filename".id ipopts fragbits dsize flags flow flowbits seq ack window itype icode icmp id icmp seq rpc ip proto sameip
The id keyword is used to check the IP ID ﬁeld for a speciﬁc value.
3.1 logto
The logto keyword tells Snort to log all packets that trigger this rule to a special output log ﬁle. version. The ﬂow keyword allows rules to only apply to certain directions of the trafﬁc ﬂow.
3. The window keyword is used to check for a speciﬁc TCP window size. or all. The seq keyword is used to check for a speciﬁc TCP sequence number. The binary keyword prints out data in a binary format. The all keyword substitutes non-printable characters with their hexadecimal equivalents.2 session
The session keyword is built to extract user data from TCP Sessions. The icmp seq keyword is used to check for a speciﬁc ICMP sequence value. rlogin.
Format
session:[printable|binary|all]. ftp.

does not properly handle tagged alerts.3 resp
The resp keyword enables an active response that kills the offending session.)
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.seconds.Tag packets containing the destination IP address of the packet that generated the initial event.Count is speciﬁed as a number of units. a tagged packet limit will be used to limit the number of tagged packets regardless of whether the seconds or bytes count has been reached.Tag the host/session for <count> packets • seconds .1.600. The default tagged packet limit value is 256 and can be modiﬁed by using a conﬁg option in your snort.tagged. • dst . <count>. metric • packets . Once a rule is triggered. so it should not be used in heavy load situations. described in Section 2.seconds.Log packets from the host that caused the tag to activate (uses [direction] modiﬁer) count • <integer> . Tagged trafﬁc is logged to allow analysis of response codes and post-attack trafﬁc.7. Subsequent tagged alerts will cause the limit to reset.
Format
tag:<type>.4 react
The react keyword enables an active response that includes sending a web page or other content to the client and then closing the connection. Note that neither subsequent alerts nor event ﬁlters will prevent a tagged packet from being logged.1 any (flowbits:isnotset. type • session . tag:host. Resp can be used in both passive or inline modes.6. additional trafﬁc involving the source and/or destination host is tagged.4 for details. The binary keyword does not log any protocol headers below the application layer.5 tag
The tag keyword allow rules to log more than just the single packet that triggered the rule.0.only relevant if host type is used.7.conf ﬁle (see Section 2.7. and Stream reassembly will cause duplicate data when the reassembled packets are logged. You can disable this packet limit for a particular rule by adding a packets metric to your tag option and setting its count to 0 (This can be done on a global scale by setting the tagged packet limit option in snort. See 2. but it is the responsibility of the output plugin to properly handle these special alerts. <metric>[. tagged alerts will be sent to the same output plugins as the original alert.1.4 any -> 10.1. direction].
3. Doing this will ensure that packets are tagged for the full amount of seconds or bytes and will not be cut off by the tagged packet limit. flowbits:set.Warnings
Using the session keyword can slow Snort down considerably.11.3 on how to use the tagged packet limit conﬁg option).11. The session keyword is best suited for post-processing binary (pcap) log ﬁles. See 2.1.6.) Also note that if you have a tag option in a rule that uses a metric other than packets. (Note that the tagged packet limit was introduced to avoid DoS situations on high bandwidth sensors for tag rules with a high seconds or bytes counts. Currently.1.3 for details. • src .src.Tag the host/session for <count> bytes direction . React can be used in both passive and inline modes.conf to 0).packets.
3. alert tcp any any <> 10.Tag packets containing the source IP address of the packet that generated the initial event. the database output plugin.1.
3.Tag the host/session for <count> seconds • bytes .) alert tcp 10. tag:host.src.600.1.1 any \ (content:"TAGMYPACKETS".Log packets in the session that set off the rule • host . Units are speciﬁed in the <metric> ﬁeld.tagged.

seconds 60. seconds <s>.7. one per content.10 detection ﬁlter
detection ﬁlter deﬁnes a rate which must be exceeded by a source or destination host before a rule can generate an event.7 activated by
The activated by keyword allows the rule writer to dynamically enable a rule when a speciﬁc activate rule is triggered. count:50. Both the new string and the content it is to replace must have the same length.7.12. At most one detection filter is permitted per rule. after evaluating all other rule options (regardless of the position of the ﬁlter within the rule source).2.6 for more information.100 22 ( \ msg:"SSH Brute Force Attempt".2.)
3.8 count
The count keyword must be used in combination with the activated by keyword.
Format
activated_by:1. \ detection_filter:track by_src. You can have multiple replacements within a rule. after the ﬁrst 30 failed login attempts: drop tcp 10.Example
This example logs the ﬁrst 10 seconds or the tagged packet limit (whichever comes ﬁrst) of any telnet session. depth:4.7.1.10.
3. alert tcp any any -> any 23 (flags:s. \ count <c>. Example . \ content:"SSH". rev:1.
Format
activates:1.9 replace
The replace keyword is a feature available in inline mode which will cause Snort to replace the prior matching content with the given string.)
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. See Section 3. replace:"<string>". count 30.7.2. \ sid:1000001. flow:established.1. offset:0.2. See Section 3. detection ﬁlter has the following format: detection_filter: \ track <by_src|by_dst>.2. Snort evaluates a detection filter as the last step of the detection phase.6 for more information.
3.100 any > 10.6 activates
The activates keyword allows the rule writer to specify a rule to add when a speciﬁc network event occurs.100 during one sampling period of 60 seconds.
3.this rule will ﬁre on every failed login attempt from 10.1.
3.seconds.1. nocase.
Format
activated_by:1.7. tag:session. It allows the rule writer to specify how many packets to leave the rule enabled for after it is activated. See Section 3.to_server.6 for more information.

Format
threshold: \ type <limit|threshold|both>.
Since potentially many events will be generated. Some rules may only make sense with a threshold. The session keyword is built to extract user data from TCP Sessions. Time period over which count is accrued. \ uricontent:"/robots. The value must be nonzero. This keyword implements an ability for users to react to trafﬁc that matches a Snort rule by closing connection and sending a notice. These should incorporate the threshold into the rule.txt".7. a rule for detecting a too many login password attempts may require more than 5 attempts. seconds <s>. There is no functional difference between adding a threshold to a rule. This means count is maintained for each unique source IP address or each unique destination IP address.
Examples
This rule logs the ﬁrst event of this SID every 60 seconds. or you can use standalone thresholds that reference the generator and SID they are applied to. C must be nonzero. threshold:type limit.Option track by src|by dst count c seconds s
Description Rate is tracked either by source IP address or destination IP address. Use detection filters (3. flow:to_server. established.txt access". Available in inline mode only. This keyword allows the rule writer to specify a rule to add when a speciﬁc network event occurs. \ count <c>. alert tcp $external_net any -> $http_servers $http_ports \ (msg:"web-misc robots. or using a standalone threshold applied to the same rule.8 Rule Thresholds
! △NOTE
Rule thresholds are deprecated and will not be supported in a future release.2) as standalone conﬁgurations instead.12: Post-detection rule option keywords Keyword logto session resp react tag activates activated by count replace detection filter Description The logto keyword tells Snort to log all packets that trigger this rule to a special output log ﬁle. There is a logical difference.
threshold can be included as part of a rule. nocase.10) within rules. reference:nessus. \ classtype:web-application-activity.7.11 Post-Detection Quick Reference
Table 3. The maximum number of rule matches in s seconds allowed before the detection ﬁlter limit to be exceeded. For instance. The resp keyword is used attempt to close sessions when an alert is triggered. track \ 178
. or event filters (2. This can be done using the ‘limit’ type of threshold.
3. \ track <by_src|by_dst>. The tag keyword allow rules to log more than just the single packet that triggered the rule.10302. This keyword must be used in combination with the activated by keyword. Replace the prior matching content with the given string of the same length.
3. It allows the rule writer to specify how many packets to leave the rule enabled for after it is activated.4. a detection filter would normally be used in conjunction with an event filter to reduce the number of logged events. This keyword allows the rule writer to dynamically enable a rule when a speciﬁc activate rule is triggered. Track by source or destination IP address and if the rule otherwise matches more than the conﬁgured rate it will ﬁre. It makes sense that the threshold feature is an integral part of this rule.

a multi-pattern matcher is used to select rules that have a chance at matching based on a single content.txt access". then by ports (ip and icmp use slightly differnet logic). especially when applied to large rule groups like HTTP. threshold:type threshold. track \ by_dst.txt". the less likely that rule and all of it’s rule options will be evaluated unnecessarily . \ track by_dst.9. established. seconds 60. the client sends: user username_here A simple rule to look for FTP root login attempts could be:
179
. to send the username. or for each unique destination IP addresses. FTP is a good example.9. By writing rules for the vulnerability. threshold:type both.9 Writing Good Rules
There are some general concepts to keep in mind when developing Snort rules to maximize efﬁciency and speed. Ports or anything else are not tracked. nocase. count 10 .)
3. flow:to_server. Type both alerts once per time interval after seeing m occurrences of the event. Selecting rules for evaluation via this ”fast” pattern matcher was found to increase performance. or destination IP address. then ignores any additional events during the time interval. instead of a speciﬁc exploit.10302. then ignores events for the rest of the time interval. Not the Exploit
Try to write rules that target the vulnerability. \ uricontent:"/robots. time period over which count is accrued. established. then by those with content and those without. udp.9. try and have at least one content (or uricontent) rule option in your rule. reference:nessus.
3. rev:1.
3. For rules with content. \ classtype:web-application-activity.10302. nocase. While some detection options.it’s safe to say there is generally more ”good” trafﬁc than ”bad”. the rule is less vulnerable to evasion when an attacker changes the exploit slightly.
alert tcp $external_net any -> $http_servers $http_ports \ (msg:"web-misc robots. they are not used by the fast pattern matching engine. This means count is maintained for each unique source IP addresses. Rules without content are always evaluated (relative to the protocol and port group in which they reside). such as pcre and byte test. number of rule matching in s seconds that will cause event filter limit to be exceeded.) This rule logs at most one event every 60 seconds if at least 10 events on this SID are ﬁred.txt access". flow:to_server. If at all possible.1 Content Matching
Snort groups rules by protocol (ip. reference:nessus.txt". \ classtype:web-application-activity.
3. sid:1000852. potentially putting a drag on performance. perform detection in the payload section of the packet. sid:1000852. The longer and more unique a content is.3 Catch the Oddities of the Protocol in the Rule
Many services typically send the commands in upper case letters. s must be nonzero value. In FTP. \ uricontent:"/robots. For example. tcp. alert tcp $external_net any -> $http_servers $http_ports \ (msg:"web-misc robots. rev:1. look for a the vulnerable command with an argument that is too large.2 Catch the Vulnerability. rate is tracked either by source IP address.Option type limit|threshold|both
track by src|by dst
count c seconds s
Description type limit alerts on the 1st m events during the time interval. c must be nonzero value. instead of shellcode that binds a shell. count 10. seconds 60 . icmp). Type threshold alerts every m times we see this event during the time interval.

as the dsize check is the ﬁrst option checked and dsize is a discrete check without recursion. even though it is obvious that the payload “aab” has “a” immediately followed by “b”. the rule needs more smarts than a simple string match. then look for the pattern again after where it was found the previous time. followed by root. the recursion implementation is not very smart.) This rule would look for “a”. By looking at this rule snippit. then the dsize option would fail.4 Optimizing Rules
The content matching portion of the detection engine has recursion to handle a few evasion cases. looking for root. because of recursion. • The rule has a pcre option. but it is needed. then check the dsize again. that may not sound like a smart idea. the content 0x13 would be found again starting after where the previous 0x13 was found. Rules that are not properly written can cause Snort to waste time duplicating checks. the following rule options are not optimized: content:"|13|".
3. take the following rule: alert ip any any -> any any (content:"a". The following rule options are discrete and should generally be placed at the beginning of any rule: • dsize • flags • flow
180
.established. and because of recursion. pcre:"/user\s+root/i". This option is added to allow the fast pattern matcher to select this rule for evaluation only if the content root is found in the payload. and if any of the detection options after that pattern fail. followed at least one space character (which includes tab). For example. \ content:"root". a good rule will handle all of the odd things that the protocol might handle when accepting the user command. ignoring case. content:"|13|".9. once it is found. For example. within:1. verifying this is trafﬁc going to the server on an established session.) There are a few important things to note in this rule: • The rule has a ﬂow option. Without recursion. Repeat until the pattern is not found again or the opt functions all succeed. A good rule that looks for root login on ftp would be: alert tcp any any -> any 21 (flow:to_server. repeating until 0x13 is not found in the payload again. However. most unique string in the attack.alert tcp any any -> any any 21 (content:"user root". • The rule has a content option. a packet with 1024 bytes of 0x13 could cause 1023 too many pattern match attempts and 1023 too many dsize checks. content:"b". Why? The content 0x13 would be found in the ﬁrst byte. immediately followed by “b”. the payload “aab” would fail. While recursion is important for detection. A packet of 1024 bytes of 0x13 would fail immediately. it is obvious the rule looks for a packet with a single byte of 0x13. For example. looking for user. The optimized rule snipping would be: dsize:1. On ﬁrst read. The way the recursion works now is if a pattern matches. Reordering the rule options so that discrete checks (such as dsize) are moved to the beginning of the rule speed up Snort. which is the longest. dsize:1.) While it may seem trivial to write a rule that looks for the username root. each of the following are accepted by most FTP servers: user root user root user root user root user<tab>root To handle all of the cases that the FTP server might handle. because the ﬁrst ”a” is not immediately followed by “b”.

content:"|00 01 87 88|". depth:4. To do that in a Snort rule.unix timestamp (0x40283a10 = 1076378128 = feb 10 01:55:28 2004 gmt) 00 00 00 0a . we need to make sure that our packet is an RPC call. This is where byte test is useful. aligning on the 4 byte boundary. depth:4. we know the vulnerability is that sadmind trusts the uid coming from the client. the data we have is: 00 00 00 0a 4d 45 54 41 53 50 4c 4f 49 54 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 We want to read 4 bytes. offset:12. content:"|00 00 00 01|". aka none) . If we do that.align. we need to make sure that our packet is a call to the procedure 1. content:"|00 00 00 01|". we need to make sure that our packet is a call to sadmind. making sure to account for the padding that RPC requires on strings. byte_jump:4.gid of requesting user (0) 00 00 00 00 . offset:20. depth:4.align. depth:4. However. depth:4. offset:16. but we want to skip over it and check a number value after the hostname. offset:4.
182
. content:"|00 00 00 01|". content:"|00 01 87 88|". offset:20. let’s put them all together. within:4.36.verifier flavor (0 = auth\_null.40 28 3a 10 . and jump that many bytes forward.metasploit 00 00 00 00 . then we want to look for the uid of 0. offset:4. offset:16. and turn those 4 bytes into an integer and jump that many bytes forward. In english. content:"|00 00 00 00|". aka none)
The rest of the packet is the request that gets passed to procedure 1 of sadmind. First. we use: byte_jump:4. within:4. content:"|00 00 00 00|". we have decoded enough of the request to write our rule. 36 bytes from the beginning of the packet. Then. content:"|00 00 00 00|". the vulnerable procedure. content:"|00 00 00 00|". content:"|00 00 00 01|". sadmind runs any request where the client’s uid is 0 as root. turn it into a number.36. Now that we have all the detection capabilities for our rule. depth:4. We don’t care about the hostname. we need to make sure that our packet has auth unix credentials. offset:12. Then. As such.length of verifier (0. Then. depth:4.length of the client machine name (0x0a = 10) 4d 45 54 41 53 50 4c 4f 49 54 00 00 . the value we want to check. we want to read 4 bytes.extra group ids (0) 00 00 00 00 00 00 00 00 . depth:4. Starting at the length of the hostname. we are now at: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 which happens to be the exact location of the uid.uid of requesting user (0) 00 00 00 00 .

The 3rd and fourth string match are right next to each other. We end up with: content:"|00 00 00 00|". within:4. offset:4. offset:12. depth:8. content:"|00 01 87 88|". we would check the length of the hostname to make sure it is not too large. byte_test:4. offset:16. In Snort. instead of reading the length of the hostname and jumping that many bytes forward. content:"|00 00 00 01 00 00 00 01|".36.36. we would read 4 bytes. depth:8. content:"|00 00 00 00|". turn it into a number. depth:4. depth:4.200. so we should combine those patterns. Our full rule would be: content:"|00 00 00 00|".36.>. content:"|00 00 00 01 00 byte_jump:4. depth:4. and then make sure it is not too large (let’s say bigger than 200 bytes). content:"|00 01 87 88|".
If the sadmind service was vulnerable to a buffer overﬂow when reading the client’s hostname.align. offset:16. To do that. we do: byte_test:4. offset:4. offset:12. 00 00 01|". depth:4.200. starting 36 bytes into the packet.>.
183
.

4. and debugging info. rules.
184
.1 Data Structures
A number of data structures are central to the API. } DynamicPluginMeta. char *libraryPath. and path to the shared library. int major. access to the StreamAPI.h. char uniqueName[MAX_NAME_LEN]. the dynamic API presents a means for loading dynamic libraries and allowing the module to utilize certain functions within the main snort code. and it provides access to the normalized http and alternate data buffers. or detection engine). restart.Chapter 4
Dynamic Modules
Preprocessors. When enabled via the –enabledynamicplugin conﬁgure option. exit. It also includes information for setting alerts. int build. It is deﬁned in sf dynamic meta. and processing functions. fatal errors. int minor. This data structure should be initialized when the preprocessor shared library is loaded. the version information. detection capabilities. handling Inline drops. This includes functions to register the preprocessor’s conﬁguration parsing. A shared library can implement all three types. errors. check the appropriate header ﬁles for the current deﬁnitions.1.
4.h as: #define MAX_NAME_LEN 1024 #define TYPE_ENGINE 0x01 #define TYPE_DETECTION 0x02 #define TYPE_PREPROCESSOR 0x04 typedef struct _DynamicPluginMeta { int type. and rules can now be developed as dynamically loadable module to snort. It includes function to log messages.2 DynamicPreprocessorData
The DynamicPreprocessorData structure deﬁnes the interface the preprocessor uses to interact with snort itself. and rules as a dynamic plugin to snort.1. The remainder of this chapter will highlight the data structures and API functions used in developing preprocessors.1 DynamicPluginMeta
The DynamicPluginMeta structure deﬁnes the type of dynamic module (preprocessor.
4. The deﬁnition of each is deﬁned in the following sections. Check the header ﬁle for the current deﬁnition. detection engines. It is deﬁned in sf dynamic preprocessor. Beware: the deﬁnitions herein may be out of date. but typically is limited to a single functionality such as a preprocessor.

GetRuleData getRuleData. This includes functions for logging messages. PCRECompileFunc pcreCompile. generator and signature IDs. LogMsgFunc logMsg. It also includes a list of rule options and an optional evaluation function. GetPreprocRuleOptFuncs getPreprocOptFuncs.
4.5 Dynamic Rules
A dynamic rule should use any of the following data structures. u_int8_t *altBuffer. revision. int *debugMsgLine. address and port information and rule information (classiﬁcation. errors. } DynamicEngineData. DetectAsn1 asn1Detect. RegisterBit flowbitRegister. PCREStudyFunc pcreStudy. It and the data structures it incorporates are deﬁned in sf snort packet.4 SFSnortPacket
The SFSnortPacket structure mirrors the snort Packet structure and provides access to all of the data contained in a given packet. UriInfo *uriBuffers[MAX_URIINFOS]. Check the header ﬁle for the current deﬁnitions. LogMsgFunc fatalMsg. RegisterRule ruleRegister. and debugging info as well as a means to register and check ﬂowbits. That includes protocol. It is deﬁned in sf dynamic engine.3 DynamicEngineData
The DynamicEngineData structure deﬁnes the interface a detection engine uses to interact with snort itself. fatal errors. PCREExecFunc pcreExec.
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.h.
4.1. priority.4.h. #endif char **debugMsgFile. #define RULE_MATCH 1 #define RULE_NOMATCH 0 typedef struct _Rule { IPInfo ip. LogMsgFunc errMsg. classiﬁcation. RuleInformation info. CheckFlowbit flowbitCheck. and it provides access to the normalized http and alternate data buffers.
Rule
The Rule structure deﬁnes the basic outline of a rule and contains the same set of information that is seen in a text rule. SetRuleData setRuleData.1.h as: typedef struct _DynamicEngineData { int version. It also includes a location to store rule-stubs for dynamic rules that are loaded. /* NULL terminated array of RuleOption union */ ruleEvalFunc evalFunc. char *dataDumpDirectory.1. The following structures are deﬁned in sf snort plugin api. and a list of references). #ifdef HAVE_WCHAR_H DebugWideMsgFunc debugWideMsg. RuleOption **options. DebugMsgFunc debugMsg. Additional data structures may be deﬁned to reference other protocol ﬁelds.

} option_u. etc. the one with the longest content length will be used. unicode. • OptionType: Content & Structure: ContentInfo The ContentInfo structure deﬁnes an option for a content search. OPTION_TYPE_BYTE_JUMP. The ”Not” ﬂag is used to negate the results of evaluating that option. and ﬂags (one of which must specify the buffer – raw. such as the compiled PCRE information. FlowBitsInfo *flowBit.RuleOption
The RuleOption structure deﬁnes a single rule option as an option type and a reference to the data speciﬁc to that option. } ContentInfo. typedef struct _ContentInfo { u_int8_t *pattern. depth and offset. OPTION_TYPE_HDR_CHECK. u_int8_t *patternByteForm. ByteData *byte. HdrOptCheck *hdrData. typedef enum DynamicOptionType { OPTION_TYPE_PREPROCESSOR. OPTION_TYPE_MAX }. OPTION_TYPE_SET_CURSOR. Each option has a ﬂags ﬁeld that contains speciﬁc ﬂags for that option as well as a ”Not” ﬂag. OPTION_TYPE_LOOP. OPTION_TYPE_FLOWFLAGS. OPTION_TYPE_BYTE_EXTRACT. OPTION_TYPE_PCRE. relative. /* must include a CONTENT_BUF_X */ void *boyer_ptr. and a designation that this content is to be used for snorts fast pattern evaluation. CursorInfo *cursor. The most unique content. u_int32_t depth. the integer ID for a ﬂowbit. OPTION_TYPE_CONTENT. URI or normalized – to search). LoopInfo *loop. union { void *ptr. } RuleOption. int32_t offset. u_int32_t flags. that which distinguishes this rule as a possible match to a packet. #define NOT_FLAG 0x10000000
Some options also contain information that is initialized at run time. OPTION_TYPE_ASN1. PCREInfo *pcre. u_int32_t patternByteFormLength. OPTION_TYPE_FLOWBIT. OPTION_TYPE_BYTE_TEST. Boyer-Moore content information. Asn1Context *asn1. PreprocessorOption *preprocOpt. should be marked for fast pattern evaluation. #define CONTENT_NOCASE #define CONTENT_RELATIVE #define CONTENT_UNICODE2BYTE 0x01 0x02 0x04
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. ContentInfo *content. typedef struct _RuleOption { int optionType. Additional ﬂags include nocase. ByteExtract *byteExtract. if no ContentInfo structure in a given rules uses that ﬂag. FlowFlags *flowFlags. It includes the pattern. The option types and related structures are listed below. OPTION_TYPE_CURSOR. In the dynamic detection engine provided with Snort. u_int32_t incrementLength.

/* void *memoryLocation. DynamicElement *increment. /* Holder */ int32_t staticInt. /* u_int32_t multiplier. /* type of this field . • OptionType: Set Cursor & Structure: CursorInfo See Cursor Check above. CursorInfo *cursorAdjust. u_int32_t value.32bits is MORE than enough */ must include a CONTENT_BUF_X */
• OptionType: Byte Jump & Structure: ByteData See Byte Test above. It includes the number of bytes. It includes a cursor adjust that happens through each iteration of the loop. DynamicElement *end. for checkValue.
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4. u_int32_t flags. } LoopInfo.
Number of bytes to extract */ Offset from cursor */ Multiply value by this (similar to byte jump) */ must include a CONTENT_BUF_X */ To match up with a DynamicElement refId */ Location to store the data extracted */
The DynamicElement structure is used to deﬁne the values for a looping evaluation. For a dynamic element. typedef struct _LoopInfo { DynamicElement *start. #define DYNAMIC_TYPE_INT_STATIC 1 #define DYNAMIC_TYPE_INT_REF 2 typedef struct _DynamicElement { char dynamicType. for checkValue */ Value to compare value against.DynamicElement The LoopInfo structure deﬁnes the information for a set of options that are to be evaluated repeatedly. /* Pointer to value of dynamic */ } data. a reference to a RuleInfo structure that deﬁnes the RuleOptions are to be evaluated through each iteration. It includes whether the element is static (an integer) or dynamic (extracted from a buffer in the packet) and the value. /* } ByteExtract. u_int32_t op. • OptionType: Loop & Structures: LoopInfo. /* char *refId. u_int32_t op.ByteExtract. /* int32_t offset. u_int32_t multiplier. /* u_int32_t flags. int32_t offset.#define CHECK_ATLEASTONE #define CHECK_NONE typedef struct _ByteData { u_int32_t bytes. */
The ByteExtract structure deﬁnes the information to use when extracting bytes for a DynamicElement used a in Loop evaltion. multiplier. specifies
* relative. or extracted value */ Offset from cursor */ Used for byte jump -.static or reference */ char *refId.
9 10
/* /* /* /* /* /*
Number of bytes to extract */ Type of byte comparison. end.
/* /* /* /* /* /* * /* /*
Starting value of FOR loop (i=start) */ Ending value of FOR loop (i OP end) */ Increment value of FOR loop (i+= increment) */ Type of comparison for loop termination */ How to move cursor each iteration of loop */ Pointer to SubRule & options to evaluate within the loop */ Loop initialized properly (safeguard) */ can be used to negate loop results. u_int32_t flags. u_int8_t initialized. /* Value of static */ int32_t *dynamicInt. } ByteData. typedef struct _ByteExtract { u_int32_t bytes. the value is ﬁlled by a related ByteExtract option that is part of the loop. One of those options may be a ByteExtract. The loop option acts like a FOR loop and includes start. } DynamicElement. and a reference to the DynamicElement. and increment values as well as the comparison operation for termination.2 Required Functions
Each dynamic module must deﬁne a set of functions and data objects to work within this framework. ﬂags specifying the buffer. an offset. /* reference ID (NULL if static) */ union { void *voidPtr. struct _Rule *subRule.

2 Detection Engine
Each dynamic detection engine library must deﬁne the following functions. – int checkCursor(void *p. – int checkValue(void *p. With a text rule. and the distance option corresponds to offset. as speciﬁed by ByteExtract and delimited by cursor. • int InitializeEngineLib(DynamicEngineData *) This function initializes the data structure for use by the engine. Cursor position is updated and returned in *cursor. – int checkFlow(void *p. • int InitializePreprocessor(DynamicPreprocessorData *) This function initializes the data structure for use by the preprocessor into a library global variable. Value extracted is stored in ByteExtract memoryLocation parameter. PCRE evalution data. ByteExtract *byteExtract. – int byteJump(void *p. – int detectAsn1(void *p.2. It will interact with ﬂowbits used by text-based rules. the with option corresponds to depth.1 check for a given packet. and register ﬂowbits.
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. u int32 t value. u int8 t *cursor) This function compares the value to the value stored in ByteData. ByteData *byteData. ByteData *byteData. as speciﬁed by FlowBitsInfo. New cursor position is returned in *cursor. u int8 t **cursor) This function adjusts the cursor as delimited by CursorInfo. The metadata and setup function for the preprocessor should be deﬁned sf preproc info. • int LibVersion(DynamicPluginMeta *) This function returns the metadata for the shared library. u int8 t *cursor) This is a wrapper for extractValue() followed by checkValue(). checking for the existence of that content as delimited by ContentInfo and cursor.h. FlowBitsInfo *ﬂowbits) This function evaluates the ﬂowbits for a given packet. Each of the functions below returns RULE MATCH if the option matches based on the current criteria (cursor position.1 Preprocessors
Each dynamic preprocessor library must deﬁne the following functions. FlowFlags *ﬂowﬂags) This function evaluates the ﬂow for a given packet. byteJump. The sample code provided with Snort predeﬁnes those functions and deﬁnes the following APIs to be used by a dynamic rules library. u int8 t *cursor) This function validates that the cursor is within bounds of the speciﬁed buffer. drop. It handles bounds checking for the speciﬁed buffer and returns RULE NOMATCH if the cursor is moved out of bounds. as delimited by Asn1Context and cursor. CursorInfo *cursorInfo.c. Asn1Context *asn1.
4. – int extractValue(void *p. PCREInfo *pcre. – int setCursor(void *p.2. – int processFlowbits(void *p. It is also used by contentMatch.4. • int ruleMatch(void *p. This uses the individual functions outlined below for each of the rule options and handles repetitive content issues. u int8 t *cursor) This function evaluates an ASN. – int pcreMatch(void *p. dpd and invokes the setup function. ContentInfo* content. Cursor position is updated and returned in *cursor. checking for the existence of the expression as delimited by PCREInfo and cursor. log. etc). initialize it to setup content searches. u int8 t *cursor) This function extracts the bytes from a given packet. Rule *rule) This is the function to evaluate a rule if the rule does not have its own Rule Evaluation Function. – int contentMatch(void *p. These are deﬁned in the ﬁle sf dynamic preproc lib. ByteData *byteData. – int byteTest(void *p. CursorInfo *cursorInfo.Rule **) This is the function to iterate through each rule in the list and write a rule-stop to be used by snort to control the action of the rule (alert. etc). u int8 t **cursor) This is a wrapper for extractValue() followed by setCursor(). • int LibVersion(DynamicPluginMeta *) This function returns the metadata for the shared library. • int DumpRules(char *. and pcreMatch to adjust the cursor position after a successful match. u int8 t **cursor) This function evaluates a single content for a given packet. • int RegisterRules(Rule **) This is the function to iterate through each rule in the list. u int8 t **cursor) This function evaluates a single pcre for a given packet.

Take extra care to handle this situation and search for the matched pattern again if subsequent rule options fail to match.– int checkHdrOpt(void *p. Examples are deﬁned in the ﬁle sfnort dynamic detection lib. deﬁned in sf preproc info. • int DumpSkeletonRules() This functions writes out the rule-stubs for rules that are loaded. as delimited by LoopInfo and cursor.c and sf dynamic preproc lib.
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.3.3 Examples
This section provides a simple example of a dynamic preprocessor and a dynamic rule. Deﬁne the Setup function to register the initialization function.c and is compiled together with sf dynamic preproc lib. as spepcifed by PreprocessorOption. LoopInfo *loop. patterns that occur more than once may result in false negatives. This preprocessor always alerts on a Packet if the TCP port matches the one conﬁgured. HdrOptCheck *optData) This function evaluates the given packet’s protocol headers. This should be done for both content and PCRE options. u int8 t **cursor) This function is used to revert to a previously saved temporary cursor position. • int EngineVersion(DynamicPluginMeta *) This function deﬁnes the version requirements for the corresponding detection engine library. #define #define #define #define MAJOR_VERSION 1 MINOR_VERSION 0 BUILD_VERSION 0 PREPROC_NAME "SF_Dynamic_Example_Preprocessor" ExampleSetup
#define DYNAMIC_PREPROC_SETUP extern void ExampleSetup(). The metadata and setup function for the preprocessor should be deﬁned in sfsnort dynamic detection lib.
If you decide to write you own rule evaluation function.
4. – int preprocOptionEval(void *p.
The remainder of the code is deﬁned in spp example.h. u int8 t **cursor) This function is used to handled repetitive contents to save off a cursor position temporarily to be reset at later point.c into lib sfdynamic preprocessor example.so. This assumes the the ﬁles sf dynamic preproc lib. u int8 t **cursor) This function evaluates the preprocessor deﬁned option.3 Rules
Each dynamic rules library must deﬁne the following functions. It should set up fast pattern-matcher content. The sample code provided with Snort predeﬁnes those functions and uses the following data within the dynamic rules library.c. – void revertTempCursor(u int8 t **temp cursor. Cursor position is updated and returned in *cursor. register ﬂowbits. PreprocessorOption *preprocOpt.1 Preprocessor Example
The following is an example of a simple preprocessor. – void setTempCursor(u int8 t **temp cursor.h. This is the metadata for this preprocessor. etc. • int LibVersion(DynamicPluginMeta *) This function returns the metadata for the shared library.
4. – int loopEval(void *p. • int InitializeDetection() This function registers each rule in the rules library.2. as speciﬁed by HdrOptCheck.
! △NOTE
4.h are used. u int8 t **cursor) This function iterates through the SubRule of LoopInfo. • Rule *rules[] A NULL terminated list of Rule structures that this library deﬁnes. Cursor position is updated and returned in *cursor.

5. This is intended to help developers get a basic understanding of whats going on quickly. Each of the keyword options is a plugin.
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5.2 Snort Data Flow
First. Check out src/decode. there are a lot of packet ﬂags available that can be used to mark a packet as “reassembled” or logged. If you are going to be helping out with Snort development.sourceforge. Packets are then sent through the detection engine.net mailing list.2. Packets are passed through a series of decoder routines that ﬁrst ﬁll out the packet structure for link level protocols then are further decoded for things like TCP and UDP ports. Features go into HEAD. look at an existing output plugin and copy it to a new item and change a few things.2. this chapter is here as a place holder. Packets are then sent through the registered set of preprocessors. It can do this by checking: if (p->tcph==null) return.Chapter 5
Snort Development
Currently.
5. We’ve had problems in the past of people submitting patches only to the stable branch (since they are likely writing this stuff for their own IDS purposes). trafﬁc is acquired from the network link via libpcap. It will someday contain references on how to create new detection plugins and preprocessors.1 Preprocessors
For example. Each preprocessor checks to see if this packet is something it should look at. please use the HEAD branch of cvs. This allows this to be easily extensible. Patches should done with the command diff -nu snort-orig snort-new. Similarly. a TCP analysis preprocessor could simply return if the packet does not have a TCP header. we’ll document what these few things are.h for the list of pkt * constants.3 Output Plugins
Generally. Bug ﬁxes are what goes into STABLE. The detection engine checks each packet against the various options listed in the Snort conﬁg ﬁles. new output plugins should go into the barnyard project rather than the Snort project. We are currently cleaning house on the available output options. Later. End users don’t really need to be reading this section.2.1 Submitting Patches
Patches to Snort should be sent to the snort-devel@lists.2 Detection Plugins
Basically.
5.
5.